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To whom is this tutorial directed?
This tutorial is for those people who want to learn programming in C++ and do not necessarily have any previous knowledge of other programming languages. Of course any knowledge of other programming languages or any general computer skill can be useful to better understand this tutorial, although it is not essential.It is also suitable for those who need a little update on the new features the language has acquired from the latests standards.
If you are familiar with the C language, you can take the first 3 parts of this tutorial as a review of concepts, since they mainly explain the C part of C++. There are slight differences in the C++ syntax for some C features, so I recommend you its reading anyway.
The 4th part describes object-oriented programming.
The 5th part mostly describes the new features introduced by ANSI-C++ standard.
Structure of this tutorial
The tutorial is divided in 6 parts and each part is divided on its turn into different sections covering a topic each one. You can access any section directly from the section index available on the left side bar, or begin the tutorial from any point and follow the links at the bottom of each section.Many sections include examples that describe the use of the newly acquired knowledge in the chapter. It is recommended to read these examples and to be able to understand each of the code lines that constitute it before passing to the next chapter.
A good way to gain experience with a programming language is by modifying and adding new functionalities on your own to the example programs that you fully understand. Don't be scared to modify the examples provided with this tutorial, that's the way to learn!
Compatibility Notes
The ANSI-C++ standard acceptation as an international standard is relatively recent. It was first published in November 1997, and revised in 2003. Nevertheless, the C++ language exists from a long time before (1980s). Therefore there are many compilers which do not support all the new capabilities included in ANSI-C++, specially those released prior to the publication of the standard.This tutorial is thought to be followed with modern compilers that suport -at least on some degree- ANSI-C++ specifications. I encourage you to get one if yours is not adapted. There are many options, both commercial and free.
Compilers
The examples included in this tutorial are all console programs. That means they use text to communicate with the user and to show their results.All C++ compilers support the compilation of console programs. Check the user's manual of your compiler for more info on how to compile them.
Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:
| // my first program in C++#include <iostream>using namespace std;int main (){ cout << "Hello World!"; return 0;} | Hello World! |
The previous program is the typical program that programmer apprentices write for the first time, and its result is the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in C++, but it already contains the fundamental components that every C++ program has. We are going to look line by line at the code we have just written:
// my first program in C++
This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is.
#include <iostream>
Lines beginning with a pound sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include <iostream> tells the preprocessor to include the iostream standard file. This specific file (iostream) includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is going to be used later in the program.
using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace, the namespace with the name std. So in order to access its functionality we declare with this expression that we will be using these entities. This line is very frequent in C++ programs that use the standard library, and in fact it will be included in most of the source codes included in these tutorials.
int main ()
This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function.
The word main is followed in the code by a pair of parentheses (()). That is because it is a function declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them.
Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.
cout << "Hello World";
This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program.
cout represents the standard output stream in C++, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen).
cout is declared in the iostream standard file within the std namespace, so that's why we needed to include that specific file and to declare that we were going to use this specific namespace earlier in our code.
Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).
return 0;
The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ program.
You may have noticed that not all the lines of this program perform actions when the code is executed. There were lines containing only comments (those beginning by //). There were lines with directives for the compiler's preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case, the main function) and, finally lines with statements (like the insertion into cout), which were all included within the block delimited by the braces ({}) of the main function.
The program has been structured in different lines in order to be more readable, but in C++, we do not have strict rules on how to separate instructions in different lines. For example, instead of
| int main (){ cout << " Hello World "; return 0;} |
| int main () { cout << "Hello World"; return 0; } |
In C++, the separation between statements is specified with an ending semicolon (;) at the end of each one, so the separation in different code lines does not matter at all for this purpose. We can write many statements per line or write a single statement that takes many code lines. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it.
Let us add an additional instruction to our first program:
| // my second program in C++#include <iostream>using namespace std;int main (){ cout << "Hello World! "; cout << "I'm a C++ program"; return 0;} | Hello World! I'm a C++ program |
| int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; } |
| int main (){ cout << "Hello World!"; cout << "I'm a C++ program"; return 0;} |
Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and discarded by the preprocessor and do not produce any code by themselves. Preprocessor directives must be specified in their own line and do not have to end with a semicolon (;).
Comments
Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.C++ supports two ways to insert comments:
| // line comment/* block comment */ |
We are going to add comments to our second program:
| /* my second program in C++ with more comments */#include <iostream>using namespace std;int main (){ cout << "Hello World! "; // prints Hello World! cout << "I'm a C++ program"; // prints I'm a C++ program return 0;} | Hello World! I'm a C++ program |
Let us think that I ask you to retain the number 5 in your mental memory, and then I ask you to memorize also the number 2 at the same time. You have just stored two different values in your memory. Now, if I ask you to add 1 to the first number I said, you should be retaining the numbers 6 (that is 5+1) and 2 in your memory. Values that we could now for example subtract and obtain 4 as result.
The whole process that you have just done with your mental memory is a simile of what a computer can do with two variables. The same process can be expressed in C++ with the following instruction set:
| a = 5;b = 2;a = a + 1;result = a - b; |
Therefore, we can define a variable as a portion of memory to store a determined value.
Each variable needs an identifier that distinguishes it from the others, for example, in the previous code the variable identifiers were a, b and result, but we could have called the variables any names we wanted to invent, as long as they were valid identifiers.
Identifiers
A valid identifier is a sequence of one or more letters, digits or underline characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. Only letters, digits and underline characters are valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline character (_ ), but this is usually reserved for compiler specific keywords or external identifiers. In no case they can begin with a digit.Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword of the C++ language or your compiler's specific ones since they could be confused with these. The standard reserved keywords are:
asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while
Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved words under some circumstances:
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq
Your compiler may also include some additional specific reserved keywords.
Very important: The C++ language is a "case sensitive" language. That means that an identifier written in capital letters is not equivalent to another one with the same name but written in small letters. Thus, for example, the RESULT variable is not the same as the result variable or the Result variable. These are three different variable identifiers.
Fundamental data types
When programming, we store the variables in our computer's memory, but the computer has to know what we want to store in them, since it is not going to occupy the same amount of memory to store a simple number than to store a single letter or a large number, and they are not going to be interpreted the same way.The memory in our computers is organized in bytes. A byte is the minimum amount of memory that we can manage in C++. A byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or non-integer numbers.
Next you have a summary of the basic fundamental data types in C++, as well as the range of values that can be represented with each one:
| Name | Description | Size* | Range* |
| char | Character or small integer. | 1byte | signed: -128 to 127 unsigned: 0 to 255 |
| short int (short) | Short Integer. | 2bytes | signed: -32768 to 32767 unsigned: 0 to 65535 |
| int | Integer. | 4bytes | signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 |
| long int (long) | Long integer. | 4bytes | signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 |
| bool | Boolean value. It can take one of two values: true or false. | 1byte | true or false |
| float | Floating point number. | 4bytes | 3.4e +/- 38 (7 digits) |
| double | Double precision floating point number. | 8bytes | 1.7e +/- 308 (15 digits) |
| long double | Long double precision floating point number. | 8bytes | 1.7e +/- 308 (15 digits) |
| wchar_t | Wide character. | 2bytes | 1 wide character |
Declaration of variables
In order to use a variable in C++, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid variable identifier. For example:| int a;float mynumber; |
If you are going to declare more than one variable of the same type, you can declare all of them in a single statement by separating their identifiers with commas. For example:
| int a, b, c; |
| int a;int b;int c; |
| unsigned short int NumberOfSisters;signed int MyAccountBalance; |
| int MyAccountBalance; |
An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed or unsigned if you intend to store numerical values in a char-sized variable.
short and long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types: short is equivalent to short int and long is equivalent to long int. The following two variable declarations are equivalent:
| short Year;short int Year; |
| unsigned NextYear;unsigned int NextYear; |
| // operating with variables#include <iostream>using namespace std;int main (){ // declaring variables: int a, b; int result; // process: a = 5; b = 2; a = a + 1; result = a - b; // print out the result: cout << result; // terminate the program: return 0;} | 4 |
Scope of variables
All the variables that we intend to use in a program must have been declared with its type specifier in an earlier point in the code, like we did in the previous code at the beginning of the body of the function main when we declared that a, b, and result were of type int.A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block.
Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.
The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa.
Initialization of variables
When declaring a regular local variable, its value is by default undetermined. But you may want a variable to store a concrete value at the same moment that it is declared. In order to do that, you can initialize the variable. There are two ways to do this in C++:The first one, known as c-like, is done by appending an equal sign followed by the value to which the variable will be initialized:
type identifier = initial_value ;
For example, if we want to declare an int variable called a initialized with a value of 0 at the moment in which it is declared, we could write:
| int a = 0; |
type identifier (initial_value) ;
For example:
| int a (0); |
| // initialization of variables#include <iostream>using namespace std;int main (){ int a=5; // initial value = 5 int b(2); // initial value = 2 int result; // initial value undetermined a = a + 3; result = a - b; cout << result; return 0;} | 6 |
Introduction to strings
Variables that can store non-numerical values that are longer than one single character are known as strings.The C++ language library provides support for strings through the standard string class. This is not a fundamental type, but it behaves in a similar way as fundamental types do in its most basic usage.
A first difference with fundamental data types is that in order to declare and use objects (variables) of this type we need to include an additional header file in our source code: <string> and have access to the std namespace (which we already had in all our previous programs thanks to the using namespace statement).
| // my first string#include <iostream>#include <string>using namespace std;int main (){ string mystring = "This is a string"; cout << mystring; return 0;} | This is a string |
| string mystring = "This is a string";string mystring ("This is a string"); |
| // my first string#include <iostream>#include <string>using namespace std;int main (){ string mystring; mystring = "This is the initial string content"; cout << mystring << endl; mystring = "This is a different string content"; cout << mystring << endl; return 0;} | This is the initial string contentThis is a different string content |
| Constants | |
Literals
Literals are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example, when we wrote:| a = 5; |
Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values. Integer Numerals
| 1776707-273 |
In addition to decimal numbers (those that all of us are used to use every day) C++ allows the use as literal constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number we have to precede it with a 0 (zero character). And in order to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:
| 75 // decimal0113 // octal0x4b // hexadecimal |
Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:
| 75 // int75u // unsigned int75l // long75ul // unsigned long |
| 3.14159 // 3.141596.02e23 // 6.02 x 10231.6e-19 // 1.6 x 10-193.0 // 3.0 |
The default type for floating point literals is double. If you explicitly want to express a float or long double numerical literal, you can use the f or l suffixes respectively:
| 3.14159L // long double6.02e23f // float |
| 'z''p'"Hello world""How do you do?" |
When writing both single character and string literals, it is necessary to put the quotation marks surrounding them to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these two expressions:
| x'x' |
Character and string literals have certain peculiarities, like the escape codes. These are special characters that are difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of them are preceded by a backslash (\). Here you have a list of some of such escape codes:
| \n | newline |
| \r | carriage return |
| \t | tab |
| \v | vertical tab |
| \b | backspace |
| \f | form feed (page feed) |
| \a | alert (beep) |
| \' | single quote (') |
| \" | double quote (") |
| \? | question mark (?) |
| \\ | backslash (\) |
| '\n''\t'"Left \t Right""one\ntwo\nthree" |
String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each unfinished line.
| "string expressed in \two lines" |
| "this forms" "a single" "string" "of characters" |
| L"This is a wide character string" |
Defined constants (#define)
You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the #define preprocessor directive. Its format is:#define identifier value
For example:
| #define PI 3.14159265#define NEWLINE '\n' |
| // defined constants: calculate circumference#include <iostream>using namespace std;#define PI 3.14159#define NEWLINE '\n';int main (){ double r=5.0; // radius double circle; circle = 2 * PI * r; cout << circle; cout << NEWLINE; return 0;} | 31.4159 |
The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the end, it will also be appended in all occurrences within the body of the program that the preprocessor replaces.
Declared constants (const)
With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:| const int pathwidth = 100;const char tabulator = '\t';const zipcode = 12440; |
You do not have to memorize all the content of this page. Most details are only provided to serve as a later reference in case you need it.
Assignment (=)
The assignment operator assigns a value to a variable.| a = 5; |
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from right to left, and never the other way:
| a = b; |
Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore a later change of b will not affect the new value of a.
For example, let us have a look at the following code - I have included the evolution of the content stored in the variables as comments:
| // assignment operator#include <iostream>using namespace std;int main (){ int a, b; // a:?, b:? a = 10; // a:10, b:? b = 4; // a:10, b:4 a = b; // a:4, b:4 b = 7; // a:4, b:7 cout << "a:"; cout << a; cout << " b:"; cout << b; return 0;} | a:4 b:7 |
A property that C++ has over other programming languages is that the assignment operation can be used as the rvalue (or part of an rvalue) for another assignment operation. For example:
| a = 2 + (b = 5); |
| b = 5;a = 2 + b; |
The following expression is also valid in C++:
| a = b = c = 5; |
Arithmetic operators ( +, -, *, /, % )
The five arithmetical operations supported by the C++ language are:| + | addition |
| - | subtraction |
| * | multiplication |
| / | division |
| % | modulo |
| a = 11 % 3; |
Compound assignment (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)
When we want to modify the value of a variable by performing an operation on the value currently stored in that variable we can use compound assignment operators:| expression | is equivalent to |
| value += increase; | value = value + increase; |
| a -= 5; | a = a - 5; |
| a /= b; | a = a / b; |
| price *= units + 1; | price = price * (units + 1); |
| // compound assignment operators#include <iostream>using namespace std;int main (){ int a, b=3; a = b; a+=2; // equivalent to a=a+2 cout << a; return 0;} | 5 |
Increase and decrease (++, --)
Shortening even more some expressions, the increase operator (++) and the decrease operator (--) increase or reduce by one the value stored in a variable. They are equivalent to +=1 and to -=1, respectively. Thus:| c++;c+=1;c=c+1; |
In the early C compilers, the three previous expressions probably produced different executable code depending on which one was used. Nowadays, this type of code optimization is generally done automatically by the compiler, thus the three expressions should produce exactly the same executable code.
A characteristic of this operator is that it can be used both as a prefix and as a suffix. That means that it can be written either before the variable identifier (++a) or after it (a++). Although in simple expressions like a++ or ++a both have exactly the same meaning, in other expressions in which the result of the increase or decrease operation is evaluated as a value in an outer expression they may have an important difference in their meaning: In the case that the increase operator is used as a prefix (++a) the value is increased before the result of the expression is evaluated and therefore the increased value is considered in the outer expression; in case that it is used as a suffix (a++) the value stored in a is increased after being evaluated and therefore the value stored before the increase operation is evaluated in the outer expression. Notice the difference:
| Example 1 | Example 2 |
| B=3; A=++B; // A contains 4, B contains 4 | B=3; A=B++; // A contains 3, B contains 4 |
Relational and equality operators ( ==, !=, >, <, >=, <= )
In order to evaluate a comparison between two expressions we can use the relational and equality operators. The result of a relational operation is a Boolean value that can only be true or false, according to its Boolean result.We may want to compare two expressions, for example, to know if they are equal or if one is greater than the other is. Here is a list of the relational and equality operators that can be used in C++:
| == | Equal to |
| != | Not equal to |
| > | Greater than |
| < | Less than |
| >= | Greater than or equal to |
| <= | Less than or equal to |
| (7 == 5) // evaluates to false.(5 > 4) // evaluates to true.(3 != 2) // evaluates to true.(6 >= 6) // evaluates to true.(5 < 5) // evaluates to false. |
| (a == 5) // evaluates to false since a is not equal to 5.(a*b >= c) // evaluates to true since (2*3 >= 6) is true. (b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false. ((b=2) == a) // evaluates to true. |
Logical operators ( !, &&, || )
The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its operand is false. Basically, it returns the opposite Boolean value of evaluating its operand. For example:| !(5 == 5) // evaluates to false because the expression at its right (5 == 5) is true. !(6 <= 4) // evaluates to true because (6 <= 4) would be false. !true // evaluates to false!false // evaluates to true. |
&& OPERATOR
| a | b | a && b |
| true | true | true |
| true | false | false |
| false | true | false |
| false | false | false |
|| OPERATOR
| a | b | a || b |
| true | true | true |
| true | false | true |
| false | true | true |
| false | false | false |
| ( (5 == 5) && (3 > 6) ) // evaluates to false ( true && false ).( (5 == 5) || (3 > 6) ) // evaluates to true ( true || false ). |
Conditional operator ( ? )
The conditional operator evaluates an expression returning a value if that expression is true and a different one if the expression is evaluated as false. Its format is:condition ? result1 : result2
If condition is true the expression will return result1, if it is not it will return result2.
| 7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5.7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2.5>3 ? a : b // returns the value of a, since 5 is greater than 3.a>b ? a : b // returns whichever is greater, a or b. |
| // conditional operator#include <iostream>using namespace std;int main (){ int a,b,c; a=2; b=7; c = (a>b) ? a : b; cout << c; return 0;} | 7 |
Comma operator ( , )
The comma operator (,) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.For example, the following code:
| a = (b=3, b+2); |
Bitwise Operators ( &, |, ^, ~, <<, >> )
Bitwise operators modify variables considering the bit patterns that represent the values they store.| operator | asm equivalent | description |
| & | AND | Bitwise AND |
| | | OR | Bitwise Inclusive OR |
| ^ | XOR | Bitwise Exclusive OR |
| ~ | NOT | Unary complement (bit inversion) |
| << | SHL | Shift Left |
| >> | SHR | Shift Right |
Explicit type casting operator
Type casting operators allow you to convert a datum of a given type to another. There are several ways to do this in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be converted by the new type enclosed between parentheses (()):| int i;float f = 3.14;i = (int) f; |
| i = int ( f ); |
sizeof()
This operator accepts one parameter, which can be either a type or a variable itself and returns the size in bytes of that type or object:| a = sizeof (char); |
The value returned by sizeof is a constant, so it is always determined before program execution.
Other operators
Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for object-oriented programming. Each one is treated in its respective section.Precedence of operators
When writing complex expressions with several operands, we may have some doubts about which operand is evaluated first and which later. For example, in this expression:| a = 5 + 7 % 2 |
| a = 5 + (7 % 2) // with a result of 6, ora = (5 + 7) % 2 // with a result of 0 |
| Level | Operator | Description | Grouping |
| 1 | :: | scope | Left-to-right |
| 2 | () [] . -> ++ -- dynamic_cast static_cast reinterpret_cast const_cast typeid | postfix | Left-to-right |
| 3 | ++ -- ~ ! sizeof new delete | unary (prefix) | Right-to-left |
| * & | indirection and reference (pointers) | ||
| + - | unary sign operator | ||
| 4 | (type) | type casting | Right-to-left |
| 5 | .* ->* | pointer-to-member | Left-to-right |
| 6 | * / % | multiplicative | Left-to-right |
| 7 | + - | additive | Left-to-right |
| 8 | << >> | shift | Left-to-right |
| 9 | < > <= >= | relational | Left-to-right |
| 10 | == != | equality | Left-to-right |
| 11 | & | bitwise AND | Left-to-right |
| 12 | ^ | bitwise XOR | Left-to-right |
| 13 | | | bitwise OR | Left-to-right |
| 14 | && | logical AND | Left-to-right |
| 15 | || | logical OR | Left-to-right |
| 16 | ?: | conditional | Right-to-left |
| 17 | = *= /= %= += -= >>= <<= &= ^= != | assignment | Right-to-left |
| 18 | , | comma | Left-to-right |
All these precedence levels for operators can be manipulated or become more legible by removing possible ambiguities using parentheses signs ( and ), as in this example:
| a = 5 + 7 % 2; |
| a = 5 + (7 % 2); |
| a = (5 + 7) % 2; |
So if you want to write complicated expressions and you are not completely sure of the precedence levels, always include parentheses. It will also become a code easier to read. Until now, the example programs of previous sections provided very little interaction with the user, if any at all. Using the standard input and output library, we will be able to interact with the user by printing messages on the screen and getting the user's input from the keyboard.
C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract characters to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentialy.
The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.
Standard Output (cout)
By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout.cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).
| cout << "Output sentence"; // prints Output sentence on screencout << 120; // prints number 120 on screencout << x; // prints the content of x on screen |
| cout << "Hello"; // prints Hellocout << Hello; // prints the content of Hello variable |
| cout << "Hello, " << "I am " << "a C++ statement"; |
| cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode; |
| Hello, I am 24 years old and my zipcode is 90064 |
| cout << "This is a sentence.";cout << "This is another sentence."; |
This is a sentence.This is another sentence.
even though we had written them in two different insertions into cout. In order to perform a line break on the output we must explicitly insert a new-line character into cout. In C++ a new-line character can be specified as \n (backslash, n):
| cout << "First sentence.\n ";cout << "Second sentence.\nThird sentence."; |
First sentence.
Second sentence.
Third sentence.
Additionally, to add a new-line, you may also use the endl manipulator. For example:
| cout << "First sentence." << endl;cout << "Second sentence." << endl; |
First sentence.
Second sentence.
The endl manipulator produces a newline character, exactly as the insertion of '\n' does, but it also has an additional behavior when it is used with buffered streams: the buffer is flushed. Anyway, cout will be an unbuffered stream in most cases, so you can generally use both the \n escape character and the endl manipulator in order to specify a new line without any difference in its behavior.
Standard Input (cin).
The standard input device is usually the keyboard. Handling the standard input in C++ is done by applying the overloaded operator of extraction (>>) on the cin stream. The operator must be followed by the variable that will store the data that is going to be extracted from the stream. For example:| int age;cin >> age; |
cin can only process the input from the keyboard once the RETURN key has been pressed. Therefore, even if you request a single character, the extraction from cin will not process the input until the user presses RETURN after the character has been introduced.
You must always consider the type of the variable that you are using as a container with cin extractions. If you request an integer you will get an integer, if you request a character you will get a character and if you request a string of characters you will get a string of characters.
| // i/o example#include <iostream>using namespace std;int main (){ int i; cout << "Please enter an integer value: "; cin >> i; cout << "The value you entered is " << i; cout << " and its double is " << i*2 << ".\n"; return 0;} | Please enter an integer value: 702The value you entered is 702 and its double is 1404. |
You can also use cin to request more than one datum input from the user:
| cin >> a >> b; |
| cin >> a;cin >> b; |
cin and strings
We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:| cin >> mystring; |
In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:
| // cin with strings#include <iostream>#include <string>using namespace std;int main (){ string mystr; cout << "What's your name? "; getline (cin, mystr); cout << "Hello " << mystr << ".\n"; cout << "What is your favorite team? "; getline (cin, mystr); cout << "I like " << mystr << " too!\n"; return 0;} | What's your name? Juan SouliéHello Juan Soulié.What is your favorite team? The IsotopesI like The Isotopes too! |
stringstream
The standard header file <sstream> defines a class called stringstream that allows a string-based object to be treated as a stream. This way we can perform extraction or insertion operations from/to strings, which is especially useful to convert strings to numerical values and vice versa. For example, if we want to extract an integer from a string we can write:| string mystr ("1204");int myint;stringstream(mystr) >> myint; |
After this piece of code, the variable myint will contain the numerical value 1204.
| // stringstreams#include <iostream>#include <string>#include <sstream>using namespace std;int main (){ string mystr; float price=0; int quantity=0; cout << "Enter price: "; getline (cin,mystr); stringstream(mystr) >> price; cout << "Enter quantity: "; getline (cin,mystr); stringstream(mystr) >> quantity; cout << "Total price: " << price*quantity << endl; return 0;} | Enter price: 22.25Enter quantity: 7Total price: 155.75 |
Using this method, instead of direct extractions of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of obtaining input from the user (we now simply ask for lines) with the interpretation of that input. Therefore, this method is usually preferred to get numerical values from the user in all programs that are intensive in user input.
| Control Structures | |
With the introduction of control structures we are going to have to introduce a new concept: the compound-statement or block. A block is a group of statements which are separated by semicolons (;) like all C++ statements, but grouped together in a block enclosed in braces: { }:
{ statement1; statement2; statement3; }
Most of the control structures that we will see in this section require a generic statement as part of its syntax. A statement can be either a simple statement (a simple instruction ending with a semicolon) or a compund statement (several instructions grouped in a block), like the one just described. In the case that we want the statement to be a simple statement, we do not need to enclose it in braces ({}). But in the case that we want the statement to be a compund statement it must be enclosed between braces ({}), forming a block.
Conditional structure: if and else
The if keyword is used to execute a statement or block only if a condition is fulfilled. Its form is:if (condition) statement
Where condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is false, statement is ignored (not executed) and the program continues right after this conditional structure.
For example, the following code fragment prints x is 100 only if the value stored in the x variable is indeed 100:
| if (x == 100) cout << "x is 100"; |
| if (x == 100){ cout << "x is "; cout << x;} |
if (condition) statement1 else statement2
For example:
| if (x == 100) cout << "x is 100";else cout << "x is not 100"; |
The if + else structures can be concatenated with the intention of verifying a range of values. The following example shows its use telling if the value currently stored in x is positive, negative or none of them (i.e. zero):
| if (x > 0) cout << "x is positive";else if (x < 0) cout << "x is negative";else cout << "x is 0"; |
Iteration structures (loops)
Loops have as purpose to repeat a statement a certain number of times or while a condition is fulfilled. The while loop Its format is:while (expression) statement
and its functionality is simply to repeat statement while the condition set in expression is true.
For example, we are going to make a program to countdown using a while-loop:
| // custom countdown using while#include <iostream>using namespace std;int main (){ int n; cout << "Enter the starting number > "; cin >> n; while (n>0) { cout << n << ", "; --n; } cout << "FIRE!\n"; return 0;} | Enter the starting number > 88, 7, 6, 5, 4, 3, 2, 1, FIRE! |
The whole process of the previous program can be interpreted according to the following script (beginning in main):
- User assigns a value to n
- The while condition is checked (n>0). At this point there are two posibilities:
* condition is true: statement is executed (to step 3)
* condition is false: ignore statement and continue after it (to step 5) - Execute statement:
cout << n << ", ";
--n;
(prints the value of n on the screen and decreases n by 1) - End of block. Return automatically to step 2
- Continue the program right after the block: print FIRE! and end program.
Of course this is such a simple action for our computer that the whole countdown is performed instantly without any practical delay between numbers. The do-while loop Its format is:
do statement while (condition);
Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated after the execution of statement instead of before, granting at least one execution of statement even if condition is never fulfilled. For example, the following example program echoes any number you enter until you enter 0.
| // number echoer#include <iostream>using namespace std;int main (){ unsigned long n; do { cout << "Enter number (0 to end): "; cin >> n; cout << "You entered: " << n << "\n"; } while (n != 0); return 0;} | Enter number (0 to end): 12345You entered: 12345Enter number (0 to end): 160277You entered: 160277Enter number (0 to end): 0You entered: 0 |
for (initialization; condition; increase) statement;
and its main function is to repeat statement while condition remains true, like the while loop. But in addition, the for loop provides specific locations to contain an initialization statement and an increase statement. So this loop is specially designed to perform a repetitive action with a counter which is initialized and increased on each iteration.
It works in the following way:
- initialization is executed. Generally it is an initial value setting for a counter variable. This is executed only once.
- condition is checked. If it is true the loop continues, otherwise the loop ends and statement is skipped (not executed).
- statement is executed. As usual, it can be either a single statement or a block enclosed in braces { }.
- finally, whatever is specified in the increase field is executed and the loop gets back to step 2.
| // countdown using a for loop#include <iostream>using namespace std;int main (){ for (int n=10; n>0; n--) { cout << n << ", "; } cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE! |
Optionally, using the comma operator (,) we can specify more than one expression in any of the fields included in a for loop, like in initialization, for example. The comma operator (,) is an expression separator, it serves to separate more than one expression where only one is generally expected. For example, suppose that we wanted to initialize more than one variable in our loop:
| for ( n=0, i=100 ; n!=i ; n++, i-- ){ // whatever here...} |
n starts with a value of 0, and i with 100, the condition is n!=i (that n is not equal to i). Because n is increased by one and i decreased by one, the loop's condition will become false after the 50th loop, when both n and i will be equal to 50.
Jump statements.
The break statement Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, we are going to stop the count down before its natural end (maybe because of an engine check failure?):| // break loop example#include <iostream>using namespace std;int main (){ int n; for (n=10; n>0; n--) { cout << n << ", "; if (n==3) { cout << "countdown aborted!"; break; } } return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, countdown aborted! |
| // continue loop example#include <iostream>using namespace std;int main (){ for (int n=10; n>0; n--) { if (n==5) continue; cout << n << ", "; } cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 4, 3, 2, 1, FIRE! |
The destination point is identified by a label, which is then used as an argument for the goto statement. A label is made of a valid identifier followed by a colon (:).
Generally speaking, this instruction has no concrete use in structured or object oriented programming aside from those that low-level programming fans may find for it. For example, here is our countdown loop using goto:
| // goto loop example#include <iostream>using namespace std;int main (){ int n=10; loop: cout << n << ", "; n--; if (n>0) goto loop; cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE! |
The purpose of exit is to terminate the current program with a specific exit code. Its prototype is:
| void exit (int exitcode); |
The selective structure: switch.
The syntax of the switch statement is a bit peculiar. Its objective is to check several possible constant values for an expression. Something similar to what we did at the beginning of this section with the concatenation of several if and else if instructions. Its form is the following:switch (expression){ case constant1: group of statements 1; break; case constant2: group of statements 2; break; . . . default: default group of statements} It works in the following way: switch evaluates expression and checks if it is equivalent to constant1, if it is, it executes group of statements 1 until it finds the break statement. When it finds this break statement the program jumps to the end of the switch selective structure.
If expression was not equal to constant1 it will be checked against constant2. If it is equal to this, it will execute group of statements 2 until a break keyword is found, and then will jump to the end of the switch selective structure.
Finally, if the value of expression did not match any of the previously specified constants (you can include as many case labels as values you want to check), the program will execute the statements included after the default: label, if it exists (since it is optional).
Both of the following code fragments have the same behavior:
| switch example | if-else equivalent |
| switch (x) { case 1: cout << "x is 1"; break; case 2: cout << "x is 2"; break; default: cout << "value of x unknown"; } | if (x == 1) { cout << "x is 1"; }else if (x == 2) { cout << "x is 2"; }else { cout << "value of x unknown"; } |
For example, if we did not include a break statement after the first group for case one, the program will not automatically jump to the end of the switch selective block and it would continue executing the rest of statements until it reaches either a break instruction or the end of the switch selective block. This makes unnecessary to include braces { } surrounding the statements for each of the cases, and it can also be useful to execute the same block of instructions for different possible values for the expression being evaluated. For example:
| switch (x) { case 1: case 2: case 3: cout << "x is 1, 2 or 3"; break; default: cout << "x is not 1, 2 nor 3"; } |
If you need to check ranges or values that are not constants, use a concatenation of if and else if statements. Using functions we can structure our programs in a more modular way, accessing all the potential that structured programming can offer to us in C++.
A function is a group of statements that is executed when it is called from some point of the program. The following is its format:
type name ( parameter1, parameter2, ...) { statement }
where:
- type is the data type specifier of the data returned by the function.
- name is the identifier by which it will be possible to call the function.
- parameters (as many as needed): Each parameter consists of a data type specifier followed by an identifier, like any regular variable declaration (for example: int x) and which acts within the function as a regular local variable. They allow to pass arguments to the function when it is called. The different parameters are separated by commas.
- statements is the function's body. It is a block of statements surrounded by braces { }.
| // function example#include <iostream>using namespace std;int addition (int a, int b){ int r; r=a+b; return (r);}int main (){ int z; z = addition (5,3); cout << "The result is " << z; return 0;} | The result is 8 |
We can see how the main function begins by declaring the variable z of type int. Right after that, we see a call to a function called addition. Paying attention we will be able to see the similarity between the structure of the call to the function and the declaration of the function itself some code lines above:
The parameters and arguments have a clear correspondence. Within the main function we called to addition passing two values: 5 and 3, that correspond to the int a and int b parameters declared for function addition.
At the point at which the function is called from within main, the control is lost by main and passed to function addition. The value of both arguments passed in the call (5 and 3) are copied to the local variables int a and int b within the function.
Function addition declares another local variable (int r), and by means of the expression r=a+b, it assigns to r the result of a plus b. Because the actual parameters passed for a and b are 5 and 3 respectively, the result is 8.
The following line of code:
| return (r); |
So being the value returned by a function the value given to the function call itself when it is evaluated, the variable z will be set to the value returned by addition (5, 3), that is 8. To explain it another way, you can imagine that the call to a function (addition (5,3)) is literally replaced by the value it returns (8).
The following line of code in main is:
| cout << "The result is " << z; |
| // function example#include <iostream>using namespace std;int subtraction (int a, int b){ int r; r=a-b; return (r);}int main (){ int x=5, y=3, z; z = subtraction (7,2); cout << "The first result is " << z << '\n'; cout << "The second result is " << subtraction (7,2) << '\n'; cout << "The third result is " << subtraction (x,y) << '\n'; z= 4 + subtraction (x,y); cout << "The fourth result is " << z << '\n'; return 0;} | The first result is 5The second result is 5The third result is 2The fourth result is 6 |
Nevertheless, if we examine function main we will see that we have made several calls to function subtraction. We have used some different calling methods so that you see other ways or moments when a function can be called.
In order to fully understand these examples you must consider once again that a call to a function could be replaced by the value that the function call itself is going to return. For example, the first case (that you should already know because it is the same pattern that we have used in previous examples):
| z = subtraction (7,2);cout << "The first result is " << z; |
| z = 5;cout << "The first result is " << z; |
| cout << "The second result is " << subtraction (7,2); |
| cout << "The second result is " << 5; |
In the case of:
| cout << "The third result is " << subtraction (x,y); |
The fourth case is more of the same. Simply note that instead of:
| z = 4 + subtraction (x,y); |
| z = subtraction (x,y) + 4; |
| z = 4 + 2;z = 2 + 4; |
Functions with no type. The use of void.
If you remember the syntax of a function declaration:type name ( argument1, argument2 ...) statement
you will see that the declaration begins with a type, that is the type of the function itself (i.e., the type of the datum that will be returned by the function with the return statement). But what if we want to return no value?
Imagine that we want to make a function just to show a message on the screen. We do not need it to return any value. In this case we should use the void type specifier for the function. This is a special specifier that indicates absence of type.
| // void function example#include <iostream>using namespace std;void printmessage (){ cout << "I'm a function!";}int main (){ printmessage (); return 0;} | I'm a function! |
| void printmessage (void){ cout << "I'm a function!";} |
What you must always remember is that the format for calling a function includes specifying its name and enclosing its parameters between parentheses. The non-existence of parameters does not exempt us from the obligation to write the parentheses. For that reason the call to printmessage is:
| printmessage (); |
| printmessage; | ||
| Functions (II) | | |
Arguments passed by value and by reference.
Until now, in all the functions we have seen, the arguments passed to the functions have been passed by value. This means that when calling a function with parameters, what we have passed to the function were copies of their values but never the variables themselves. For example, suppose that we called our first function addition using the following code:| int x=5, y=3, z;z = addition ( x , y ); |
This way, when the function addition is called, the value of its local variables a and b become 5 and 3 respectively, but any modification to either a or b within the function addition will not have any effect in the values of x and y outside it, because variables x and y were not themselves passed to the function, but only copies of their values at the moment the function was called.
But there might be some cases where you need to manipulate from inside a function the value of an external variable. For that purpose we can use arguments passed by reference, as in the function duplicate of the following example:
| // passing parameters by reference#include <iostream>using namespace std;void duplicate (int& a, int& b, int& c){ a*=2; b*=2; c*=2;}int main (){ int x=1, y=3, z=7; duplicate (x, y, z); cout << "x=" << x << ", y=" << y << ", z=" << z; return 0;} | x=2, y=6, z=14 |
When a variable is passed by reference we are not passing a copy of its value, but we are somehow passing the variable itself to the function and any modification that we do to the local variables will have an effect in their counterpart variables passed as arguments in the call to the function.
To explain it in another way, we associate a, b and c with the arguments passed on the function call (x, y and z) and any change that we do on a within the function will affect the value of x outside it. Any change that we do on b will affect y, and the same with c and z.
That is why our program's output, that shows the values stored in x, y and z after the call to duplicate, shows the values of all the three variables of main doubled.
If when declaring the following function:
| void duplicate (int& a, int& b, int& c) |
| void duplicate (int a, int b, int c) |
Passing by reference is also an effective way to allow a function to return more than one value. For example, here is a function that returns the previous and next numbers of the first parameter passed.
| // more than one returning value#include <iostream>using namespace std;void prevnext (int x, int& prev, int& next){ prev = x-1; next = x+1;}int main (){ int x=100, y, z; prevnext (x, y, z); cout << "Previous=" << y << ", Next=" << z; return 0;} | Previous=99, Next=101 |
Default values in parameters.
When declaring a function we can specify a default value for each parameter. This value will be used if the corresponding argument is left blank when calling to the function. To do that, we simply have to use the assignment operator and a value for the arguments in the function declaration. If a value for that parameter is not passed when the function is called, the default value is used, but if a value is specified this default value is ignored and the passed value is used instead. For example:| // default values in functions#include <iostream>using namespace std;int divide (int a, int b=2){ int r; r=a/b; return (r);}int main (){ cout << divide (12); cout << endl; cout << divide (20,4); return 0;} | 65 |
| divide (12) |
In the second call:
| divide (20,4) |
Overloaded functions.
In C++ two different functions can have the same name if their parameter types or number are different. That means that you can give the same name to more than one function if they have either a different number of parameters or different types in their parameters. For example:| // overloaded function#include <iostream>using namespace std;int operate (int a, int b){ return (a*b);}float operate (float a, float b){ return (a/b);}int main (){ int x=5,y=2; float n=5.0,m=2.0; cout << operate (x,y); cout << "\n"; cout << operate (n,m); cout << "\n"; return 0;} | 102.5 |
In the first call to operate the two arguments passed are of type int, therefore, the function with the first prototype is called; This function returns the result of multiplying both parameters. While the second call passes two arguments of type float, so the function with the second prototype is called. This one has a different behavior: it divides one parameter by the other. So the behavior of a call to operate depends on the type of the arguments passed because the function has been overloaded.
Notice that a function cannot be overloaded only by its return type. At least one of its parameters must have a different type.
inline functions.
The inline specifier is an optimization specifier. It does not change the behavior of a function, but serves to indicate the compiler that the code the function body generates shall be inserted at the point of each call to the function, instead of doing a regular call that generally involves stacking variables and jumping to other memory locations. This is equivalent to declaring a macro. It only represents an overhead advantage for very short functions, in which the resulting code from compiling the program may be faster when the overhead required to call a function is avoided.The format for its declaration is:
inline type name ( arguments ... ) { instructions ... }
and the call is just like the call to any other function. You do not have to include the inline keyword when calling the function, only in its declaration.
Most compilers already optimize code to generate inline functions when it is more convenient. This specifier only indicates the compiler that inline is preferred for this function.
Recursivity.
Recursivity is the property that functions have to be called by themselves. It is useful for many tasks, like sorting or calculate the factorial of numbers. For example, to obtain the factorial of a number (n!) the mathematical formula would be:n! = n * (n-1) * (n-2) * (n-3) ... * 1 more concretely, 5! (factorial of 5) would be:
5! = 5 * 4 * 3 * 2 * 1 = 120 and a recursive function to calculate this in C++ could be:
| // factorial calculator#include <iostream>using namespace std;long factorial (long a){ if (a > 1) return (a * factorial (a-1)); else return (1);}int main (){ long number; cout << "Please type a number: "; cin >> number; cout << number << "! = " << factorial (number); return 0;} | Please type a number: 99! = 362880 |
This function has a limitation because of the data type we used in its design (long) for more simplicity. The results given will not be valid for values much greater than 10! or 15!, depending on the system you compile it.
Declaring functions.
Until now, we have defined all of the functions before the first appearance of calls to them in the source code. These calls were generally in function main which we have always left at the end of the source code. If you try to repeat some of the examples of functions described so far, but placing the function main before any of the other functions that were called from within it, you will most likely obtain compiling errors. The reason is that to be able to call a function it must have been declared in some earlier point of the code, like we have done in all our examples.But there is an alternative way to avoid writing the whole code of a function before it can be used in main or in some other function. This can be achieved by declaring just a prototype of the function before it is used, instead of the entire definition. This declaration is shorter than the entire definition, but significant enough for the compiler to determine its return type and the types of its parameters.
Its form is:
type name ( argument_type1, argument_type2, ...);
It is identical to a function definition, except that it does not include the body of the function itself (i.e., the function statements that in normal definitions are enclosed in braces { }) and instead of that we end the prototype declaration with a mandatory semicolon (;).
The parameter enumeration does not need to include the identifiers, but only the type specifiers. The inclusion of a name for each parameter as in the function definition is optional in the prototype declaration. For example, we can declare a function called protofunction with two int parameters with any of the following declarations:
| int protofunction (int first, int second);int protofunction (int, int); |
| // declaring functions prototypes#include <iostream>using namespace std;void odd (int a);void even (int a);int main (){ int i; do { cout << "Type a number (0 to exit): "; cin >> i; odd (i); } while (i!=0); return 0;}void odd (int a){ if ((a%2)!=0) cout << "Number is odd.\n"; else even (a);}void even (int a){ if ((a%2)==0) cout << "Number is even.\n"; else odd (a);} | Type a number (0 to exit): 9Number is odd.Type a number (0 to exit): 6Number is even.Type a number (0 to exit): 1030Number is even.Type a number (0 to exit): 0Number is even. |
The first things that we see are the declaration of functions odd and even:
| void odd (int a);void even (int a); |
Anyway, the reason why this program needs at least one of the functions to be declared before it is defined is because in odd there is a call to even and in even there is a call to odd. If none of the two functions had been previously declared, a compilarion error would happen, since either odd would not not be visible from even (because it has still not been declared), or even would not be visible from odd (for the same reason).
Having the prototype of all functions together in the same place within the source code is found practical by some programmers, and this can be easily achieved by declaring all functions prototypes at the beginning of a program.
| Arrays | |
That means that, for example, we can store 5 values of type int in an array without having to declare 5 different variables, each one with a different identifier. Instead of that, using an array we can store 5 different values of the same type, int for example, with a unique identifier.
For example, an array to contain 5 integer values of type int called billy could be represented like this:
where each blank panel represents an element of the array, that in this case are integer values of type int. These elements are numbered from 0 to 4 since in arrays the first index is always 0, independently of its length.
Like a regular variable, an array must be declared before it is used. A typical declaration for an array in C++ is:
type name [elements];
where type is a valid type (like int, float...), name is a valid identifier and the elements field (which is always enclosed in square brackets []), specifies how many of these elements the array has to contain.
Therefore, in order to declare an array called billy as the one shown in the above diagram it is as simple as:
| int billy [5]; |
Initializing arrays.
When declaring a regular array of local scope (within a function, for example), if we do not specify otherwise, its elements will not be initialized to any value by default, so their content will be undetermined until we store some value in them. The elements of global and static arrays, on the other hand, are automatically initialized with their default values, which for all fundamental types this means they are filled with zeros.In both cases, local and global, when we declare an array, we have the possibility to assign initial values to each one of its elements by enclosing the values in braces { }. For example:
| int billy [5] = { 16, 2, 77, 40, 12071 }; |
The amount of values between braces { } must not be larger than the number of elements that we declare for the array between square brackets [ ]. For example, in the example of array billy we have declared that it has 5 elements and in the list of initial values within braces { } we have specified 5 values, one for each element.
When an initialization of values is provided for an array, C++ allows the possibility of leaving the square brackets empty [ ]. In this case, the compiler will assume a size for the array that matches the number of values included between braces { }:
| int billy [] = { 16, 2, 77, 40, 12071 }; |
Accessing the values of an array.
In any point of a program in which an array is visible, we can access the value of any of its elements individually as if it was a normal variable, thus being able to both read and modify its value. The format is as simple as:name[index]
Following the previous examples in which billy had 5 elements and each of those elements was of type int, the name which we can use to refer to each element is the following:
For example, to store the value 75 in the third element of billy, we could write the following statement:
| billy[2] = 75; |
| a = billy[2]; |
Notice that the third element of billy is specified billy[2], since the first one is billy[0], the second one is billy[1], and therefore, the third one is billy[2]. By this same reason, its last element is billy[4]. Therefore, if we write billy[5], we would be accessing the sixth element of billy and therefore exceeding the size of the array.
In C++ it is syntactically correct to exceed the valid range of indices for an array. This can create problems, since accessing out-of-range elements do not cause compilation errors but can cause runtime errors. The reason why this is allowed will be seen further ahead when we begin to use pointers.
At this point it is important to be able to clearly distinguish between the two uses that brackets [ ] have related to arrays. They perform two different tasks: one is to specify the size of arrays when they are declared; and the second one is to specify indices for concrete array elements. Do not confuse these two possible uses of brackets [ ] with arrays.
| int billy[5]; // declaration of a new arraybilly[2] = 75; // access to an element of the array. |
Some other valid operations with arrays:
| billy[0] = a;billy[a] = 75;b = billy [a+2];billy[billy[a]] = billy[2] + 5; |
| // arrays example#include <iostream>using namespace std;int billy [] = {16, 2, 77, 40, 12071};int n, result=0;int main (){ for ( n=0 ; n<5 ; n++ ) { result += billy[n]; } cout << result; return 0;} | 12206 |
Multidimensional arrays
Multidimensional arrays can be described as "arrays of arrays". For example, a bidimensional array can be imagined as a bidimensional table made of elements, all of them of a same uniform data type.jimmy represents a bidimensional array of 3 per 5 elements of type int. The way to declare this array in C++ would be:
| int jimmy [3][5]; |
| jimmy[1][3] |
(remember that array indices always begin by zero).
Multidimensional arrays are not limited to two indices (i.e., two dimensions). They can contain as many indices as needed. But be careful! The amount of memory needed for an array rapidly increases with each dimension. For example:
| char century [100][365][24][60][60]; |
Multidimensional arrays are just an abstraction for programmers, since we can obtain the same results with a simple array just by putting a factor between its indices:
| int jimmy [3][5]; // is equivalent toint jimmy [15]; // (3 * 5 = 15) |
| multidimensional array | pseudo-multidimensional array |
| #define WIDTH 5#define HEIGHT 3int jimmy [HEIGHT][WIDTH];int n,m;int main (){ for (n=0;n<HEIGHT;n++) for (m=0;m<WIDTH;m++) { jimmy[n][m]=(n+1)*(m+1); } return 0;} | #define WIDTH 5#define HEIGHT 3int jimmy [HEIGHT * WIDTH];int n,m;int main (){ for (n=0;n<HEIGHT;n++) for (m=0;m<WIDTH;m++) { jimmy[n*WIDTH+m]=(n+1)*(m+1); } return 0;} |
We have used "defined constants" (#define) to simplify possible future modifications of the program. For example, in case that we decided to enlarge the array to a height of 4 instead of 3 it could be done simply by changing the line:
| #define HEIGHT 3 |
| #define HEIGHT 4 |
Arrays as parameters
At some moment we may need to pass an array to a function as a parameter. In C++ it is not possible to pass a complete block of memory by value as a parameter to a function, but we are allowed to pass its address. In practice this has almost the same effect and it is a much faster and more efficient operation.In order to accept arrays as parameters the only thing that we have to do when declaring the function is to specify in its parameters the element type of the array, an identifier and a pair of void brackets []. For example, the following function:
| void procedure (int arg[]) |
| int myarray [40]; |
| procedure (myarray); |
| // arrays as parameters#include <iostream>using namespace std;void printarray (int arg[], int length) { for (int n=0; n<length; n++) cout << arg[n] << " "; cout << "\n";}int main (){ int firstarray[] = {5, 10, 15}; int secondarray[] = {2, 4, 6, 8, 10}; printarray (firstarray,3); printarray (secondarray,5); return 0;} | 5 10 152 4 6 8 10 |
In a function declaration it is also possible to include multidimensional arrays. The format for a tridimensional array parameter is:
| base_type[][depth][depth] |
| void procedure (int myarray[][3][4]) |
Arrays, both simple or multidimensional, passed as function parameters are a quite common source of errors for novice programmers. I recommend the reading of the chapter about Pointers for a better understanding on how arrays operate.
For example, the following array:
Therefore, in this array, in theory, we can store sequences of characters up to 20 characters long. But we can also store shorter sequences. For example, jenny could store at some point in a program either the sequence "Hello" or the sequence "Merry christmas", since both are shorter than 20 characters. Therefore, since the array of characters can store shorter sequences than its total length, a special character is used to signal the end of the valid sequence: the null character, whose literal constant can be written as '\0' (backslash, zero). Our array of 20 elements of type char, called jenny, can be represented storing the characters sequences "Hello" and "Merry Christmas" as: Notice how after the valid content a null character ('\0') has been included in order to indicate the end of the sequence. The panels in gray color represent char elements with undetermined values. Initialization of null-terminated character sequencesBecause arrays of characters are ordinary arrays they follow all their same rules. For example, if we want to initialize an array of characters with some predetermined sequence of characters we can do it just like any other array:
But arrays of char elements have an additional method to initialize their values: using string literals. In the expressions we have used in some examples in previous chapters, constants that represent entire strings of characters have already showed up several times. These are specified enclosing the text to become a string literal between double quotes ("). For example:
Double quoted strings (") are literal constants whose type is in fact a null-terminated array of characters. So string literals enclosed between double quotes always have a null character ('\0') automatically appended at the end. Therefore we can initialize the array of char elements called myword with a null-terminated sequence of characters by either one of these two methods:
Please notice that we are talking about initializing an array of characters in the moment it is being declared, and not about assigning values to them once they have already been declared. In fact because this type of null-terminated arrays of characters are regular arrays we have the same restrictions that we have with any other array, so we are not able to copy blocks of data with an assignment operation. Assuming mytext is a char[] variable, expressions within a source code like:
Using null-terminated sequences of charactersNull-terminated sequences of characters are the natural way of treating strings in C++, so they can be used as such in many procedures. In fact, regular string literals have this type (char[]) and can also be used in most cases.For example, cin and cout support null-terminated sequences as valid containers for sequences of characters, so they can be used directly to extract strings of characters from cin or to insert them into cout. For example:
Finally, sequences of characters stored in char arrays can easily be converted into string objects just by using the assignment operator:
| |||||||||||||
| Pointers | | ||||||||||||
The memory of your computer can be imagined as a succession of memory cells, each one of the minimal size that computers manage (one byte). These single-byte memory cells are numbered in a consecutive way, so as, within any block of memory, every cell has the same number as the previous one plus one.
This way, each cell can be easily located in the memory because it has a unique address and all the memory cells follow a successive pattern. For example, if we are looking for cell 1776 we know that it is going to be right between cells 1775 and 1777, exactly one thousand cells after 776 and exactly one thousand cells before cell 2776.
Reference operator (&)
As soon as we declare a variable, the amount of memory needed is assigned for it at a specific location in memory (its memory address). We generally do not actively decide the exact location of the variable within the panel of cells that we have imagined the memory to be - Fortunately, that is a task automatically performed by the operating system during runtime. However, in some cases we may be interested in knowing the address where our variable is being stored during runtime in order to operate with relative positions to it.The address that locates a variable within memory is what we call a reference to that variable. This reference to a variable can be obtained by preceding the identifier of a variable with an ampersand sign (&), known as reference operator, and which can be literally translated as "address of". For example:
| ted = &andy; |
For now on we are going to assume that andy is placed during runtime in the memory address 1776. This number (1776) is just and arbitrary assumption we are inventing right now in order to help clarify some concepts in this tutorial, but in reality, we cannot know before runtime the real value the address of a variable will have in memory.
Consider the following code fragment:
| andy = 25;fred = andy;ted = &andy; |
First, we have assigned the value 25 to andy (a variable whose address in memory we have assumed to be 1776).
The second statement copied to fred the content of variable andy (which is 25). This is a standard assignment operation, as we have done so many times before.
Finally, the third statement copies to ted not the value contained in andy but a reference to it (i.e., its address, which we have assumed to be 1776). The reason is that in this third assignment operation we have preceded the identifier andy with the reference operator (&), so we were no longer referring to the value of andy but to its reference (its address in memory).
The variable that stores the reference to another variable (like ted in the previous example) is what we call a pointer. Pointers are a very powerful feature of the C++ language that has many uses in advanced programming. Farther ahead, we will see how this type of variable is used and declared.
Dereference operator (*)
We have just seen that a variable which stores a reference to another variable is called a pointer. Pointers are said to "point to" the variable whose reference they store.Using a pointer we can directly access the value stored in the variable which it points to. To do this, we simply have to precede the pointer's identifier with an asterisk (*), which acts as dereference operator and that can be literally translated to "value pointed by".
Therefore, following with the values of the previous example, if we write:
| beth = *ted; |
You must clearly differentiate that the expression ted refers to the value 1776, while *ted (with an asterisk * preceding the identifier) refers to the value stored at address 1776, which in this case is 25. Notice the difference of including or not including the dereference operator (I have included an explanatory commentary of how each of these two expressions could be read):
| beth = ted; // beth equal to ted ( 1776 )beth = *ted; // beth equal to value pointed by ted ( 25 ) |
- & is the reference operator and can be read as "address of"
- * is the dereference operator and can be read as "value pointed by"
Earlier we performed the following two assignment operations:
| andy = 25;ted = &andy; |
| andy == 25&andy == 1776ted == 1776*ted == 25 |
So, after all that, you may also infer that for as long as the address pointed by ted remains unchanged the following expression will also be true:
| *ted == andy |
Declaring variables of pointer types
Due to the ability of a pointer to directly refer to the value that it points to, it becomes necessary to specify in its declaration which data type a pointer is going point to. It is not the same thing to point to a char than to point to an int or a float.The declaration of pointers follows this format:
type * name;
where type is the data type of the value that the pointer is intended to point to. This type is not the type of the pointer itself! but the type of the data the pointer points to. For example:
| int * number;char * character;float * greatnumber; |
I want to emphasize that the asterisk sign (*) that we use when declaring a pointer only means that it is a pointer (it is part of its type compound specifier), and should not be confused with the dereference operator that we have seen a bit earlier, but which is also written with an asterisk (*). They are simply two different things represented with the same sign.
Now have a look at this code:
| // my first pointer#include <iostream>using namespace std;int main (){ int firstvalue, secondvalue; int * mypointer; mypointer = &firstvalue; *mypointer = 10; mypointer = &secondvalue; *mypointer = 20; cout << "firstvalue is " << firstvalue << endl; cout << "secondvalue is " << secondvalue << endl; return 0;} | firstvalue is 10secondvalue is 20 |
First, we have assigned as value of mypointer a reference to firstvalue using the reference operator (&). And then we have assigned the value 10 to the memory location pointed by mypointer, that because at this moment is pointing to the memory location of firstvalue, this in fact modifies the value of firstvalue.
In order to demonstrate that a pointer may take several different values during the same program I have repeated the process with secondvalue and that same pointer, mypointer.
Here is an example a little bit more elaborated:
| // more pointers#include <iostream>using namespace std;int main (){ int firstvalue = 5, secondvalue = 15; int * p1, * p2; p1 = &firstvalue; // p1 = address of firstvalue p2 = &secondvalue; // p2 = address of secondvalue *p1 = 10; // value pointed by p1 = 10 *p2 = *p1; // value pointed by p2 = value pointed by p1 p1 = p2; // p1 = p2 (value of pointer is copied) *p1 = 20; // value pointed by p1 = 20 cout << "firstvalue is " << firstvalue << endl; cout << "secondvalue is " << secondvalue << endl; return 0;} | firstvalue is 10secondvalue is 20 |
Notice that there are expressions with pointers p1 and p2, both with and without dereference operator (*). The meaning of an expression using the dereference operator (*) is very different from one that does not: When this operator precedes the pointer name, the expression refers to the memory location pointed by it, while when a pointer name appears without this operator, it refers to the value of the pointer itself, that is, a reference.
Another thing that may call your attention is the line:
| int * p1, * p2; |
Otherwise, the type for the second variable declared in that line would have been int (and not int*) because of precedence relationships. If we had written:
| int * p1, p2; |
Pointers and arrays
The concept of array is very much bound to the one of pointer. In fact, the identifier of an array is equivalent to the address of its first element, as a pointer is equivalent to the address of the first element that it points to, so in fact they are the same concept. For example, supposing these two declarations:| int numbers [20];int * p; |
| p = numbers; |
| numbers = p; |
Due to the characteristics of variables, all expressions that include pointers in the following example are perfectly valid:
| // more pointers#include <iostream>using namespace std;int main (){ int numbers[5]; int * p; p = numbers; *p = 10; p++; *p = 20; p = &numbers[2]; *p = 30; p = numbers + 3; *p = 40; p = numbers; *(p+4) = 50; for (int n=0; n<5; n++) cout << numbers[n] << ", "; return 0;} | 10, 20, 30, 40, 50, |
| a[5] = 0; // a [offset of 5] = 0*(a+5) = 0; // pointed by (a+5) = 0 |
Pointer initialization
When declaring pointers we may want to explicitly specify which variable we want them to point to:| int number;int *tommy = &number; |
| int number;int *tommy;tommy = &number; |
| int number;int *tommy;*tommy = &number; |
As in the case of arrays, the compiler allows the special case that we want to initialize the content at which the pointer points with constants at the same moment the pointer is declared:
| char * terry = "hello"; |
It is important to indicate that terry contains the value 1702, and not 'h' nor "hello", although 1702 indeed is the address of both of these.
The pointer terry points to a sequence of characters and can be read as if it was an array (remember that an array is just like a constant pointer). For example, we can access the fifth element of the array with any of these two expression:
| *(terry+4)terry[4] |
Pointer arithmetics
To conduct arithmetical operations on pointers is a little different than to conduct them on regular integer data types. To begin with, only addition and subtraction operations are allowed to be conducted with them, the others make no sense in the world of pointers. But both addition and subtraction have a different behavior with pointers according to the size of the data type to which they point.When we saw the different fundamental data types, we saw that some occupy more or less space than others in the memory. For example, let's assume that in a given compiler for a specific machine, char takes 1 byte, short takes 2 bytes and long takes 4.
Suppose that we define three pointers in this compiler:
| char *mychar;short *myshort;long *mylong; |
So if we write:
| mychar++;myshort++;mylong++; |
This is applicable both when adding and subtracting any number to a pointer. It would happen exactly the same if we write:
| mychar = mychar + 1;myshort = myshort + 1;mylong = mylong + 1; |
| *p++ |
Notice the difference with:
(*p)++
Here, the expression would have been evaluated as the value pointed by p increased by one. The value of p (the pointer itself) would not be modified (what is being modified is what it is being pointed to by this pointer).
If we write:
| *p++ = *q++; |
| *p = *q;++p;++q; |
Pointers to pointers
C++ allows the use of pointers that point to pointers, that these, in its turn, point to data (or even to other pointers). In order to do that, we only need to add an asterisk (*) for each level of reference in their declarations:| char a;char * b;char ** c;a = 'z';b = &a;c = &b; |
The value of each variable is written inside each cell; under the cells are their respective addresses in memory.
The new thing in this example is variable c, which can be used in three different levels of indirection, each one of them would correspond to a different value:
- c has type char** and a value of 8092
- *c has type char* and a value of 7230
- **c has type char and a value of 'z'
void pointers
The void type of pointer is a special type of pointer. In C++, void represents the absence of type, so void pointers are pointers that point to a value that has no type (and thus also an undetermined length and undetermined dereference properties).This allows void pointers to point to any data type, from an integer value or a float to a string of characters. But in exchange they have a great limitation: the data pointed by them cannot be directly dereferenced (which is logical, since we have no type to dereference to), and for that reason we will always have to change the type of the void pointer to some other pointer type that points to a concrete data type before dereferencing it. This is done by performing type-castings.
One of its uses may be to pass generic parameters to a function:
| // increaser#include <iostream>using namespace std;void increase (void* data, int size){ switch (size) { case sizeof(char) : (*((char*)data))++; break; case sizeof(int) : (*((int*)data))++; break; }}int main (){ char a = 'x'; int b = 1602; increase (&a,sizeof(a)); increase (&b,sizeof(b)); cout << a << ", " << b << endl; return 0;} | y, 1603 |
Null pointer
A null pointer is a regular pointer of any pointer type which has a special value that indicates that it is not pointing to any valid reference or memory address. This value is the result of type-casting the integer value zero to any pointer type.| int * p;p = 0; // p has a null pointer value |
Pointers to functions
C++ allows operations with pointers to functions. The typical use of this is for passing a function as an argument to another function, since these cannot be passed dereferenced. In order to declare a pointer to a function we have to declare it like the prototype of the function except that the name of the function is enclosed between parentheses () and an asterisk (*) is inserted before the name:| // pointer to functions#include <iostream>using namespace std;int addition (int a, int b){ return (a+b); }int subtraction (int a, int b){ return (a-b); }int operation (int x, int y, int (*functocall)(int,int)){ int g; g = (*functocall)(x,y); return (g);}int main (){ int m,n; int (*minus)(int,int) = subtraction; m = operation (7, 5, addition); n = operation (20, m, minus); cout <<n; return 0;} | 8 |
| int (* minus)(int,int) = subtraction; |
| Dynamic Memory | |
The answer is dynamic memory, for which C++ integrates the operators new and delete.
Operators new and new[]
In order to request dynamic memory we use the operator new. new is followed by a data type specifier and -if a sequence of more than one element is required- the number of these within brackets []. It returns a pointer to the beginning of the new block of memory allocated. Its form is:pointer = new type
pointer = new type [number_of_elements]
The first expression is used to allocate memory to contain one single element of type type. The second one is used to assign a block (an array) of elements of type type, where number_of_elements is an integer value representing the amount of these. For example:
| int * bobby;bobby = new int [5]; |
The first element pointed by bobby can be accessed either with the expression bobby[0] or the expression *bobby. Both are equivalent as has been explained in the section about pointers. The second element can be accessed either with bobby[1] or *(bobby+1) and so on...
You could be wondering the difference between declaring a normal array and assigning dynamic memory to a pointer, as we have just done. The most important difference is that the size of an array has to be a constant value, which limits its size to what we decide at the moment of designing the program, before its execution, whereas the dynamic memory allocation allows us to assign memory during the execution of the program (runtime) using any variable or constant value as its size.
The dynamic memory requested by our program is allocated by the system from the memory heap. However, computer memory is a limited resource, and it can be exhausted. Therefore, it is important to have some mechanism to check if our request to allocate memory was successful or not.
C++ provides two standard methods to check if the allocation was successful:
One is by handling exceptions. Using this method an exception of type bad_alloc is thrown when the allocation fails. Exceptions are a powerful C++ feature explained later in these tutorials. But for now you should know that if this exception is thrown and it is not handled by a specific handler, the program execution is terminated.
This exception method is the default method used by new, and is the one used in a declaration like:
| bobby = new int [5]; // if it fails an exception is thrown |
This method can be specified by using a special object called nothrow as parameter for new:
| bobby = new (nothrow) int [5]; |
| int * bobby;bobby = new (nothrow) int [5];if (bobby == 0) { // error assigning memory. Take measures. }; |
Operator delete and delete[]
Since the necessity of dynamic memory is usually limited to specific moments within a program, once it is no longer needed it should be freed so that the memory becomes available again for other requests of dynamic memory. This is the purpose of the operator delete, whose format is:| delete pointer;delete [] pointer; |
The value passed as argument to delete must be either a pointer to a memory block previously allocated with new, or a null pointer (in the case of a null pointer, delete produces no effect).
| // rememb-o-matic#include <iostream>using namespace std;int main (){ int i,n; int * p; cout << "How many numbers would you like to type? "; cin >> i; p= new (nothrow) int[i]; if (p == 0) cout << "Error: memory could not be allocated"; else { for (n=0; n<i; n++) { cout << "Enter number: "; cin >> p[n]; } cout << "You have entered: "; for (n=0; n<i; n++) cout << p[n] << ", "; delete[] p; } return 0;} | How many numbers would you like to type? 5Enter number : 75Enter number : 436Enter number : 1067Enter number : 8Enter number : 32You have entered: 75, 436, 1067, 8, 32, |
| p= new (nothrow) int[i]; |
It is a good practice to always check if a dynamic memory block was successfully allocated. Therefore, if you use the nothrow method, you should always check the value of the pointer returned. Otherwise, use the exception method, even if you do not handle the exception. This way, the program will terminate at that point without causing the unexpected results of continuing executing a code that assumes a block of memory to have been allocated when in fact it has not.
Dynamic memory in ANSI-C
Operators new and delete are exclusive of C++. They are not available in the C language. But using pure C language, dynamic memory can also be used through the functions malloc, calloc, realloc and free, defined in the <cstdlib> header file, and since C++ is a superset of C, these functions are also available to C++ programmers (see cstdlib for more info).The memory blocks allocated by these functions are not necessarily compatible with those returned by new, so each one should be manipulated with its own set of functions or operators. We have already learned how groups of sequential data can be used in C++. But this is somewhat restrictive, since in many occasions what we want to store are not mere sequences of elements all of the same data type, but sets of different elements with different data types.
Data structures
A data structure is a group of data elements grouped together under one name. These data elements, known as members, can have different types and different lengths. Data structures are declared in C++ using the following syntax:struct structure_name {
member_type1 member_name1;
member_type2 member_name2;
member_type3 member_name3;
.
.
} object_names;
where structure_name is a name for the structure type, object_name can be a set of valid identifiers for objects that have the type of this structure. Within braces { } there is a list with the data members, each one is specified with a type and a valid identifier as its name.
The first thing we have to know is that a data structure creates a new type: Once a data structure is declared, a new type with the identifier specified as structure_name is created and can be used in the rest of the program as if it was any other type. For example:
| struct product { int weight; float price;} ;product apple;product banana, melon; |
Once declared, product has become a new valid type name like the fundamental ones int, char or short and from that point on we are able to declare objects (variables) of this compound new type, like we have done with apple, banana and melon.
Right at the end of the struct declaration, and before the ending semicolon, we can use the optional field object_name to directly declare objects of the structure type. For example, we can also declare the structure objects apple, banana and melon at the moment we define the data structure type this way:
| struct product { int weight; float price;} apple, banana, melon; |
Once we have declared our three objects of a determined structure type (apple, banana and melon) we can operate directly with their members. To do that we use a dot (.) inserted between the object name and the member name. For example, we could operate with any of these elements as if they were standard variables of their respective types:
| apple.weightapple.pricebanana.weightbanana.pricemelon.weightmelon.price |
Let's see a real example where you can see how a structure type can be used in the same way as fundamental types:
| // example about structures#include <iostream>#include <string>#include <sstream>using namespace std;struct movies_t { string title; int year;} mine, yours;void printmovie (movies_t movie);int main (){ string mystr; mine.title = "2001 A Space Odyssey"; mine.year = 1968; cout << "Enter title: "; getline (cin,yours.title); cout << "Enter year: "; getline (cin,mystr); stringstream(mystr) >> yours.year; cout << "My favorite movie is:\n "; printmovie (mine); cout << "And yours is:\n "; printmovie (yours); return 0;}void printmovie (movies_t movie){ cout << movie.title; cout << " (" << movie.year << ")\n";} | Enter title: AlienEnter year: 1979My favorite movie is: 2001 A Space Odyssey (1968)And yours is: Alien (1979) |
The objects mine and yours can also be treated as valid variables of type movies_t, for example we have passed them to the function printmovie as we would have done with regular variables. Therefore, one of the most important advantages of data structures is that we can either refer to their members individually or to the entire structure as a block with only one identifier.
Data structures are a feature that can be used to represent databases, especially if we consider the possibility of building arrays of them:
| // array of structures#include <iostream>#include <string>#include <sstream>using namespace std;#define N_MOVIES 3struct movies_t { string title; int year;} films [N_MOVIES];void printmovie (movies_t movie);int main (){ string mystr; int n; for (n=0; n<N_MOVIES; n++) { cout << "Enter title: "; getline (cin,films[n].title); cout << "Enter year: "; getline (cin,mystr); stringstream(mystr) >> films[n].year; } cout << "\nYou have entered these movies:\n"; for (n=0; n<N_MOVIES; n++) printmovie (films[n]); return 0;}void printmovie (movies_t movie){ cout << movie.title; cout << " (" << movie.year << ")\n";} | Enter title: Blade RunnerEnter year: 1982Enter title: MatrixEnter year: 1999Enter title: Taxi DriverEnter year: 1976 You have entered these movies:Blade Runner (1982)Matrix (1999)Taxi Driver (1976) |
Pointers to structures
Like any other type, structures can be pointed by its own type of pointers:| struct movies_t { string title; int year;};movies_t amovie;movies_t * pmovie; |
| pmovie = &amovie; |
We will now go with another example that includes pointers, which will serve to introduce a new operator: the arrow operator (->):
| // pointers to structures#include <iostream>#include <string>#include <sstream>using namespace std;struct movies_t { string title; int year;};int main (){ string mystr; movies_t amovie; movies_t * pmovie; pmovie = &amovie; cout << "Enter title: "; getline (cin, pmovie->title); cout << "Enter year: "; getline (cin, mystr); (stringstream) mystr >> pmovie->year; cout << "\nYou have entered:\n"; cout << pmovie->title; cout << " (" << pmovie->year << ")\n"; return 0;} | Enter title: Invasion of the body snatchersEnter year: 1978 You have entered:Invasion of the body snatchers (1978) |
| pmovie->title |
| (*pmovie).title |
| *pmovie.title |
| *(pmovie.title) |
| Expression | What is evaluated | Equivalent |
| a.b | Member b of object a | |
| a->b | Member b of object pointed by a | (*a).b |
| *a.b | Value pointed by member b of object a | *(a.b) |
Nesting structures
Structures can also be nested so that a valid element of a structure can also be on its turn another structure.| struct movies_t { string title; int year;};struct friends_t { string name; string email; movies_t favorite_movie; } charlie, maria;friends_t * pfriends = &charlie; |
| charlie.namemaria.favorite_movie.titlecharlie.favorite_movie.yearpfriends->favorite_movie.year |
| Other Data Types | |
Defined data types (typedef)
C++ allows the definition of our own types based on other existing data types. We can do this using the keyword typedef, whose format is:typedef existing_type new_type_name ;
where existing_type is a C++ fundamental or compound type and new_type_name is the name for the new type we are defining. For example:
| typedef char C;typedef unsigned int WORD;typedef char * pChar;typedef char field [50]; |
| C mychar, anotherchar, *ptc1;WORD myword;pChar ptc2;field name; |
typedef can be useful to define an alias for a type that is frequently used within a program. It is also useful to define types when it is possible that we will need to change the type in later versions of our program, or if a type you want to use has a name that is too long or confusing.
Unions
Unions allow one same portion of memory to be accessed as different data types, since all of them are in fact the same location in memory. Its declaration and use is similar to the one of structures but its functionality is totally different:union union_name { member_type1 member_name1; member_type2 member_name2; member_type3 member_name3; . .} object_names; All the elements of the union declaration occupy the same physical space in memory. Its size is the one of the greatest element of the declaration. For example:
| union mytypes_t { char c; int i; float f; } mytypes; |
| mytypes.cmytypes.imytypes.f |
One of the uses a union may have is to unite an elementary type with an array or structures of smaller elements. For example:
| union mix_t { long l; struct { short hi; short lo; } s; char c[4];} mix; |
The exact alignment and order of the members of a union in memory is platform dependant. Therefore be aware of possible portability issues with this type of use.
Anonymous unions
In C++ we have the option to declare anonymous unions. If we declare a union without any name, the union will be anonymous and we will be able to access its members directly by their member names. For example, look at the difference between these two structure declarations:| structure with regular union | structure with anonymous union |
| struct { char title[50]; char author[50]; union { float dollars; int yens; } price;} book; | struct { char title[50]; char author[50]; union { float dollars; int yens; };} book; |
| book.price.dollarsbook.price.yens |
| book.dollarsbook.yens |
Enumerations (enum)
Enumerations create new data types to contain something different that is not limited to the values fundamental data types may take. Its form is the following:enum enumeration_name { value1, value2, value3, . .} object_names; For example, we could create a new type of variable called color to store colors with the following declaration:
| enum colors_t {black, blue, green, cyan, red, purple, yellow, white}; |
| colors_t mycolor; mycolor = blue;if (mycolor == green) mycolor = red; |
We can explicitly specify an integer value for any of the constant values that our enumerated type can take. If the constant value that follows it is not given an integer value, it is automatically assumed the same value as the previous one plus one. For example:
| enum months_t { january=1, february, march, april, may, june, july, august, september, october, november, december} y2k; |
| Classes (I) | |
An object is an instantiation of a class. In terms of variables, a class would be the type, and an object would be the variable.
Classes are generally declared using the keyword class, with the following format:
class class_name { access_specifier_1: member1; access_specifier_2: member2; ... } object_names; Where class_name is a valid identifier for the class, object_names is an optional list of names for objects of this class. The body of the declaration can contain members, that can be either data or function declarations, and optionally access specifiers.
All is very similar to the declaration on data structures, except that we can now include also functions and members, but also this new thing called access specifier. An access specifier is one of the following three keywords: private, public or protected. These specifiers modify the access rights that the members following them acquire:
- private members of a class are accessible only from within other members of the same class or from their friends.
- protected members are accessible from members of their same class and from their friends, but also from members of their derived classes.
- Finally, public members are accessible from anywhere where the object is visible.
| class CRectangle { int x, y; public: void set_values (int,int); int area (void); } rect; |
Notice the difference between the class name and the object name: In the previous example, CRectangle was the class name (i.e., the type), whereas rect was an object of type CRectangle. It is the same relationship int and a have in the following declaration:
| int a; |
After the previous declarations of CRectangle and rect, we can refer within the body of the program to any of the public members of the object rect as if they were normal functions or normal variables, just by putting the object's name followed by a dot (.) and then the name of the member. All very similar to what we did with plain data structures before. For example:
| rect.set_values (3,4);myarea = rect.area(); |
Here is the complete example of class CRectangle:
| // classes example#include <iostream>using namespace std;class CRectangle { int x, y; public: void set_values (int,int); int area () {return (x*y);}};void CRectangle::set_values (int a, int b) { x = a; y = b;}int main () { CRectangle rect; rect.set_values (3,4); cout << "area: " << rect.area(); return 0;} | area: 12 |
You may notice that the definition of the member function area() has been included directly within the definition of the CRectangle class given its extreme simplicity, whereas set_values() has only its prototype declared within the class, but its definition is outside it. In this outside declaration, we must use the operator of scope (::) to specify that we are defining a function that is a member of the class CRectangle and not a regular global function.
The scope operator (::) specifies the class to which the member being declared belongs, granting exactly the same scope properties as if this function definition was directly included within the class definition. For example, in the function set_values() of the previous code, we have been able to use the variables x and y, which are private members of class CRectangle, which means they are only accessible from other members of their class.
The only difference between defining a class member function completely within its class and to include only the prototype and later its definition, is that in the first case the function will automatically be considered an inline member function by the compiler, while in the second it will be a normal (not-inline) class member function, which in fact supposes no difference in behavior.
Members x and y have private access (remember that if nothing else is said, all members of a class defined with keyword class have private access). By declaring them private we deny access to them from anywhere outside the class. This makes sense, since we have already defined a member function to set values for those members within the object: the member function set_values(). Therefore, the rest of the program does not need to have direct access to them. Perhaps in a so simple example as this, it is difficult to see an utility in protecting those two variables, but in greater projects it may be very important that values cannot be modified in an unexpected way (unexpected from the point of view of the object).
One of the greater advantages of a class is that, as any other type, we can declare several objects of it. For example, following with the previous example of class CRectangle, we could have declared the object rectb in addition to the object rect:
| // example: one class, two objects#include <iostream>using namespace std;class CRectangle { int x, y; public: void set_values (int,int); int area () {return (x*y);}};void CRectangle::set_values (int a, int b) { x = a; y = b;}int main () { CRectangle rect, rectb; rect.set_values (3,4); rectb.set_values (5,6); cout << "rect area: " << rect.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0;} | rect area: 12rectb area: 30 |
Notice that the call to rect.area() does not give the same result as the call to rectb.area(). This is because each object of class CRectangle has its own variables x and y, as they, in some way, have also their own function members set_value() and area() that each uses its object's own variables to operate.
That is the basic concept of object-oriented programming: Data and functions are both members of the object. We no longer use sets of global variables that we pass from one function to another as parameters, but instead we handle objects that have their own data and functions embedded as members. Notice that we have not had to give any parameters in any of the calls to rect.area or rectb.area. Those member functions directly used the data members of their respective objects rect and rectb.
Constructors and destructors
Objects generally need to initialize variables or assign dynamic memory during their process of creation to become operative and to avoid returning unexpected values during their execution. For example, what would happen if in the previous example we called the member function area() before having called function set_values()? Probably we would have gotten an undetermined result since the members x and y would have never been assigned a value.In order to avoid that, a class can include a special function called constructor, which is automatically called whenever a new object of this class is created. This constructor function must have the same name as the class, and cannot have any return type; not even void.
We are going to implement CRectangle including a constructor:
| // example: class constructor#include <iostream>using namespace std;class CRectangle { int width, height; public: CRectangle (int,int); int area () {return (width*height);}};CRectangle::CRectangle (int a, int b) { width = a; height = b;}int main () { CRectangle rect (3,4); CRectangle rectb (5,6); cout << "rect area: " << rect.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0;} | rect area: 12rectb area: 30 |
Notice how these arguments are passed to the constructor at the moment at which the objects of this class are created:
| CRectangle rect (3,4);CRectangle rectb (5,6); |
You can also see how neither the constructor prototype declaration (within the class) nor the latter constructor definition include a return value; not even void.
The destructor fulfills the opposite functionality. It is automatically called when an object is destroyed, either because its scope of existence has finished (for example, if it was defined as a local object within a function and the function ends) or because it is an object dynamically assigned and it is released using the operator delete.
The destructor must have the same name as the class, but preceded with a tilde sign (~) and it must also return no value.
The use of destructors is especially suitable when an object assigns dynamic memory during its lifetime and at the moment of being destroyed we want to release the memory that the object was allocated.
| // example on constructors and destructors#include <iostream>using namespace std;class CRectangle { int *width, *height; public: CRectangle (int,int); ~CRectangle (); int area () {return (*width * *height);}};CRectangle::CRectangle (int a, int b) { width = new int; height = new int; *width = a; *height = b;}CRectangle::~CRectangle () { delete width; delete height;}int main () { CRectangle rect (3,4), rectb (5,6); cout << "rect area: " << rect.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0;} | rect area: 12rectb area: 30 |
Overloading Constructors
Like any other function, a constructor can also be overloaded with more than one function that have the same name but different types or number of parameters. Remember that for overloaded functions the compiler will call the one whose parameters match the arguments used in the function call. In the case of constructors, which are automatically called when an object is created, the one executed is the one that matches the arguments passed on the object declaration:| // overloading class constructors#include <iostream>using namespace std;class CRectangle { int width, height; public: CRectangle (); CRectangle (int,int); int area (void) {return (width*height);}};CRectangle::CRectangle () { width = 5; height = 5;}CRectangle::CRectangle (int a, int b) { width = a; height = b;}int main () { CRectangle rect (3,4); CRectangle rectb; cout << "rect area: " << rect.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0;} | rect area: 12rectb area: 25 |
Important: Notice how if we declare a new object and we want to use its default constructor (the one without parameters), we do not include parentheses ():
| CRectangle rectb; // rightCRectangle rectb(); // wrong! |
Default constructor
If you do not declare any constructors in a class definition, the compiler assumes the class to have a default constructor with no arguments. Therefore, after declaring a class like this one:| class CExample { public: int a,b,c; void multiply (int n, int m) { a=n; b=m; c=a*b; }; }; |
| CExample ex; |
| class CExample { public: int a,b,c; CExample (int n, int m) { a=n; b=m; }; void multiply () { c=a*b; }; }; |
| CExample ex (2,3); |
| CExample ex; |
But the compiler not only creates a default constructor for you if you do not specify your own. It provides three special member functions in total that are implicitly declared if you do not declare your own. These are the copy constructor, the copy assignment operator, and the default destructor.
The copy constructor and the copy assignment operator copy all the data contained in another object to the data members of the current object. For CExample, the copy constructor implicitly declared by the compiler would be something similar to:
| CExample::CExample (const CExample& rv) { a=rv.a; b=rv.b; c=rv.c; } |
| CExample ex (2,3);CExample ex2 (ex); // copy constructor (data copied from ex) |
Pointers to classes
It is perfectly valid to create pointers that point to classes. We simply have to consider that once declared, a class becomes a valid type, so we can use the class name as the type for the pointer. For example:| CRectangle * prect; |
As it happened with data structures, in order to refer directly to a member of an object pointed by a pointer we can use the arrow operator (->) of indirection. Here is an example with some possible combinations:
| // pointer to classes example#include <iostream>using namespace std;class CRectangle { int width, height; public: void set_values (int, int); int area (void) {return (width * height);}};void CRectangle::set_values (int a, int b) { width = a; height = b;}int main () { CRectangle a, *b, *c; CRectangle * d = new CRectangle[2]; b= new CRectangle; c= &a; a.set_values (1,2); b->set_values (3,4); d->set_values (5,6); d[1].set_values (7,8); cout << "a area: " << a.area() << endl; cout << "*b area: " << b->area() << endl; cout << "*c area: " << c->area() << endl; cout << "d[0] area: " << d[0].area() << endl; cout << "d[1] area: " << d[1].area() << endl; return 0;} | a area: 2*b area: 12*c area: 2d[0] area: 30d[1] area: 56 |
| expression | can be read as |
| *x | pointed by x |
| &x | address of x |
| x.y | member y of object x |
| x->y | member y of object pointed by x |
| (*x).y | member y of object pointed by x (equivalent to the previous one) |
| x[0] | first object pointed by x |
| x[1] | second object pointed by x |
| x[n] | (n+1)th object pointed by x |
Classes defined with struct and union
Classes can be defined not only with keyword class, but also with keywords struct and union.The concepts of class and data structure are so similar that both keywords have in C++ the exact same functionality except that the members of classes declared with keyword struct have public access by default, instead of private access, as classes declared with keyword class have. That is the only difference. For all other purposes both keywords are equivalent.
The concept under unions is different from than of struct and class, since unions only store one data member at a time, but they are also classes and thus can also hold function members. The default access in union classes is public.
| Classes (II) | |
Overloading operators
C++ incorporates the option to use standard operators to perform operations with classes in addition to with fundamental types. For example:| int a, b, c;a = b + c; |
| struct { string product; float price;} a, b, c;a = b + c; |
| Overloadable operators |
| + - * / = < > += -= *= /= << >><<= >>= == != <= >= ++ -- % & ^ ! |~ &= ^= |= && || %= [] () , ->* -> new delete new[] delete[] |
type operator sign (parameters) { /*...*/ }
Here you have an example that overloads the addition operator (+). We are going to create a class to store bidimensional vectors and then we are going to add two of them: a(3,1) and b(1,2). The addition of two bidimensional vectors is an operation as simple as adding the two x coordinates to obtain the resulting x coordinate and adding the two y coordinates to obtain the resulting y. In this case the result will be (3+1,1+2) = (4,3).
| // vectors: overloading operators example#include <iostream>using namespace std;class CVector { public: int x,y; CVector () {}; CVector (int,int); CVector operator + (CVector);};CVector::CVector (int a, int b) { x = a; y = b;}CVector CVector::operator+ (CVector param) { CVector temp; temp.x = x + param.x; temp.y = y + param.y; return (temp);}int main () { CVector a (3,1); CVector b (1,2); CVector c; c = a + b; cout << c.x << "," << c.y; return 0;} | 4,3 |
| CVector (int, int); // function name CVector (constructor)CVector operator+ (CVector); // function returns a CVector |
| c = a + b;c = a.operator+ (b); |
Notice also that we have included the empty constructor (without parameters) and we have defined it with an empty block:
| CVector () { }; |
| CVector (int, int); |
| CVector c; |
Anyway, I have to warn you that an empty block is a bad implementation for a constructor, since it does not fulfill the minimum functionality that is generally expected from a constructor, which is the initialization of all the member variables in its class. In our case this constructor leaves the variables x and y undefined. Therefore, a more advisable definition would have been something similar to this:
| CVector () { x=0; y=0; }; |
As well as a class includes a default constructor and a copy constructor even if they are not declared, it also includes a default definition for the assignment operator (=) with the class itself as parameter. The behavior which is defined by default is to copy the whole content of the data members of the object passed as argument (the one at the right side of the sign) to the one at the left side:
| CVector d (2,3);CVector e;e = d; // copy assignment operator |
The overload of operators does not force its operation to bear a relation to the mathematical or usual meaning of the operator, although it is recommended. For example, the code may not be very intuitive if you use operator + to subtract two classes or operator== to fill with zeros a class, although it is perfectly possible to do so.
Although the prototype of a function operator+ can seem obvious since it takes what is at the right side of the operator as the parameter for the operator member function of the object at its left side, other operators may not be so obvious. Here you have a table with a summary on how the different operator functions have to be declared (replace @ by the operator in each case):
| Expression | Operator | Member function | Global function |
| @a | + - * & ! ~ ++ -- | A::operator@() | operator@(A) |
| a@ | ++ -- | A::operator@(int) | operator@(A,int) |
| a@b | + - * / % ^ & | < > == != <= >= << >> && || , | A::operator@ (B) | operator@(A,B) |
| a@b | = += -= *= /= %= ^= &= |= <<= >>= [] | A::operator@ (B) | - |
| a(b, c...) | () | A::operator() (B, C...) | - |
| a->x | -> | A::operator->() | - |
You can see in this panel that there are two ways to overload some class operators: as a member function and as a global function. Its use is indistinct, nevertheless I remind you that functions that are not members of a class cannot access the private or protected members of that class unless the global function is its friend (friendship is explained later).
The keyword this
The keyword this represents a pointer to the object whose member function is being executed. It is a pointer to the object itself.One of its uses can to check if a parameter passed to a member function is the object itself. For example,
| // this#include <iostream>using namespace std;class CDummy { public: int isitme (CDummy& param);};int CDummy::isitme (CDummy& param){ if (¶m == this) return true; else return false;}int main () { CDummy a; CDummy* b = &a; if ( b->isitme(a) ) cout << "yes, &a is b"; return 0;} | yes, &a is b |
| CVector& CVector::operator= (const CVector& param){ x=param.x; y=param.y; return *this;} |
Static members
A class can contain static members, either data or functions.Static data members of a class are also known as "class variables", because there is only one unique value for all the objects of that same class. Their content is not different from one object of this class to another.
For example, it may be used for a variable within a class that can contain a counter with the number of objects of that class that have been created, as in the following example:
| // static members in classes#include <iostream>using namespace std;class CDummy { public: static int n; CDummy () { n++; }; ~CDummy () { n--; };};int CDummy::n=0;int main () { CDummy a; CDummy b[5]; CDummy * c = new CDummy; cout << a.n << endl; delete c; cout << CDummy::n << endl; return 0;} | 76 |
| int CDummy::n=0; |
| cout << a.n;cout << CDummy::n; |
Once again, I remind you that in fact it is a global variable. The only difference is its name and possible access restrictions outside its class.
Just as we may include static data within a class, we can also include static functions. They represent the same: they are global functions that are called as if they were object members of a given class. They can only refer to static data, in no case to non-static members of the class, as well as they do not allow the use of the keyword this, since it makes reference to an object pointer and these functions in fact are not members of any object but direct members of the class.
| Statement: | Explained in: |
| int a::b(c) {}; | Classes |
| a->b | Pointers |
| class a: public b; | Friendship and inheritance |
Pointers to base class
One of the key features of derived classes is that a pointer to a derived class is type-compatible with a pointer to its base class. Polymorphism is the art of taking advantage of this simple but powerful and versatile feature, that brings Object Oriented Methodologies to its full potential.We are going to start by rewriting our program about the rectangle and the triangle of the previous section taking into consideration this pointer compatibility property:
| // pointers to base class#include <iostream>using namespace std;class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } };class CRectangle: public CPolygon { public: int area () { return (width * height); } };class CTriangle: public CPolygon { public: int area () { return (width * height / 2); } };int main () { CRectangle rect; CTriangle trgl; CPolygon * ppoly1 = ▭ CPolygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0;} | 2010 |
The only limitation in using *ppoly1 and *ppoly2 instead of rect and trgl is that both *ppoly1 and *ppoly2 are of type CPolygon* and therefore we can only use these pointers to refer to the members that CRectangle and CTriangle inherit from CPolygon. For that reason when we call the area() members at the end of the program we have had to use directly the objects rect and trgl instead of the pointers *ppoly1 and *ppoly2.
In order to use area() with the pointers to class CPolygon, this member should also have been declared in the class CPolygon, and not only in its derived classes, but the problem is that CRectangle and CTriangle implement different versions of area, therefore we cannot implement it in the base class. This is when virtual members become handy:
Virtual members
A member of a class that can be redefined in its derived classes is known as a virtual member. In order to declare a member of a class as virtual, we must precede its declaration with the keyword virtual:| // virtual members#include <iostream>using namespace std;class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area () { return (0); } };class CRectangle: public CPolygon { public: int area () { return (width * height); } };class CTriangle: public CPolygon { public: int area () { return (width * height / 2); } };int main () { CRectangle rect; CTriangle trgl; CPolygon poly; CPolygon * ppoly1 = ▭ CPolygon * ppoly2 = &trgl; CPolygon * ppoly3 = &poly; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly3->set_values (4,5); cout << ppoly1->area() << endl; cout << ppoly2->area() << endl; cout << ppoly3->area() << endl; return 0;} | 20100 |
The member function area() has been declared as virtual in the base class because it is later redefined in each derived class. You can verify if you want that if you remove this virtual keyword from the declaration of area() within CPolygon, and then you run the program the result will be 0 for the three polygons instead of 20, 10 and 0. That is because instead of calling the corresponding area() function for each object (CRectangle::area(), CTriangle::area() and CPolygon::area(), respectively), CPolygon::area() will be called in all cases since the calls are via a pointer whose type is CPolygon*.
Therefore, what the virtual keyword does is to allow a member of a derived class with the same name as one in the base class to be appropriately called from a pointer, and more precisely when the type of the pointer is a pointer to the base class but is pointing to an object of the derived class, as in the above example.
A class that declares or inherits a virtual function is called a polymorphic class.
Note that despite of its virtuality, we have also been able to declare an object of type CPolygon and to call its own area() function, which always returns 0.
Abstract base classes
Abstract base classes are something very similar to our CPolygon class of our previous example. The only difference is that in our previous example we have defined a valid area() function with a minimal functionality for objects that were of class CPolygon (like the object poly), whereas in an abstract base classes we could leave that area() member function without implementation at all. This is done by appending =0 (equal to zero) to the function declaration.An abstract base CPolygon class could look like this:
| // abstract class CPolygonclass CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area () =0;}; |
The main difference between an abstract base class and a regular polymorphic class is that because in abstract base classes at least one of its members lacks implementation we cannot create instances (objects) of it.
But a class that cannot instantiate objects is not totally useless; We can create pointers to it and take advantage of all its polymorphic abilities. Therefore a declaration like:
| CPolygon poly; |
| CPolygon * ppoly1;CPolygon * ppoly2; |
This is so for as long as CPolygon includes a pure virtual function and therefore it's an abstract base class. However, pointers to this abstract base class can be used to point to objects of derived classes.
Here you have the complete example:
| // abstract base class#include <iostream>using namespace std;class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area (void) =0; };class CRectangle: public CPolygon { public: int area (void) { return (width * height); } };class CTriangle: public CPolygon { public: int area (void) { return (width * height / 2); } };int main () { CRectangle rect; CTriangle trgl; CPolygon * ppoly1 = ▭ CPolygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); cout << ppoly1->area() << endl; cout << ppoly2->area() << endl; return 0;} | 2010 |
| // pure virtual members can be called// from the abstract base class#include <iostream>using namespace std;class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area (void) =0; void printarea (void) { cout << this->area() << endl; } };class CRectangle: public CPolygon { public: int area (void) { return (width * height); } };class CTriangle: public CPolygon { public: int area (void) { return (width * height / 2); } };int main () { CRectangle rect; CTriangle trgl; CPolygon * ppoly1 = ▭ CPolygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly1->printarea(); ppoly2->printarea(); return 0;} | 2010 |
Let's end with the same example again, but this time with objects that are dynamically allocated:
| // dynamic allocation and polymorphism#include <iostream>using namespace std;class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area (void) =0; void printarea (void) { cout << this->area() << endl; } };class CRectangle: public CPolygon { public: int area (void) { return (width * height); } };class CTriangle: public CPolygon { public: int area (void) { return (width * height / 2); } };int main () { CPolygon * ppoly1 = new CRectangle; CPolygon * ppoly2 = new CTriangle; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly1->printarea(); ppoly2->printarea(); delete ppoly1; delete ppoly2; return 0;} | 2010 |
| CPolygon * ppoly1 = new CRectangle;CPolygon * ppoly2 = new CTriangle; |
| Templates | |
Function templates
Function templates are special functions that can operate with generic types. This allows us to create a function template whose functionality can be adapted to more than one type or class without repeating the entire code for each type.In C++ this can be achieved using template parameters. A template parameter is a special kind of parameter that can be used to pass a type as argument: just like regular function parameters can be used to pass values to a function, template parameters allow to pass also types to a function. These function templates can use these parameters as if they were any other regular type.
The format for declaring function templates with type parameters is:
template <class identifier> function_declaration;
template <typename identifier> function_declaration;
The only difference between both prototypes is the use of either the keyword class or the keyword typename. Its use is indistinct, since both expressions have exactly the same meaning and behave exactly the same way.
For example, to create a template function that returns the greater one of two objects we could use:
| template <class myType>myType GetMax (myType a, myType b) { return (a>b?a:b);} |
To use this function template we use the following format for the function call:
function_name <type> (parameters);
For example, to call GetMax to compare two integer values of type int we can write:
| int x,y;GetMax <int> (x,y); |
Here is the entire example:
| // function template#include <iostream>using namespace std;template <class T>T GetMax (T a, T b) { T result; result = (a>b)? a : b; return (result);}int main () { int i=5, j=6, k; long l=10, m=5, n; k=GetMax<int>(i,j); n=GetMax<long>(l,m); cout << k << endl; cout << n << endl; return 0;} | 610 |
In the example above we used the function template GetMax() twice. The first time with arguments of type int and the second one with arguments of type long. The compiler has instantiated and then called each time the appropriate version of the function.
As you can see, the type T is used within the GetMax() template function even to declare new objects of that type:
| T result; |
In this specific case where the generic type T is used as a parameter for GetMax the compiler can find out automatically which data type has to instantiate without having to explicitly specify it within angle brackets (like we have done before specifying <int> and <long>). So we could have written instead:
| int i,j;GetMax (i,j); |
| // function template II#include <iostream>using namespace std;template <class T>T GetMax (T a, T b) { return (a>b?a:b);}int main () { int i=5, j=6, k; long l=10, m=5, n; k=GetMax(i,j); n=GetMax(l,m); cout << k << endl; cout << n << endl; return 0;} | 610 |
Because our template function includes only one template parameter (class T) and the function template itself accepts two parameters, both of this T type, we cannot call our function template with two objects of different types as arguments:
| int i;long l;k = GetMax (i,l); |
We can also define function templates that accept more than one type parameter, simply by specifying more template parameters between the angle brackets. For example:
| template <class T, class U>T GetMin (T a, U b) { return (a<b?a:b);} |
| int i,j;long l;i = GetMin<int,long> (j,l); |
| i = GetMin (j,l); |
Class templates
We also have the possibility to write class templates, so that a class can have members that use template parameters as types. For example:| template <class T>class mypair { T values [2]; public: mypair (T first, T second) { values[0]=first; values[1]=second; }}; |
| mypair<int> myobject (115, 36); |
| mypair<double> myfloats (3.0, 2.18); |
| // class templates#include <iostream>using namespace std;template <class T>class mypair { T a, b; public: mypair (T first, T second) {a=first; b=second;} T getmax ();};template <class T>T mypair<T>::getmax (){ T retval; retval = a>b? a : b; return retval;}int main () { mypair <int> myobject (100, 75); cout << myobject.getmax(); return 0;} | 100 |
| template <class T>T mypair<T>::getmax () |
Template specialization
If we want to define a different implementation for a template when a specific type is passed as template parameter, we can declare a specialization of that template.For example, let's suppose that we have a very simple class called mycontainer that can store one element of any type and that it has just one member function called increase, which increases its value. But we find that when it stores an element of type char it would be more convenient to have a completely different implementation with a function member uppercase, so we decide to declare a class template specialization for that type:
| // template specialization#include <iostream>using namespace std;// class template:template <class T>class mycontainer { T element; public: mycontainer (T arg) {element=arg;} T increase () {return ++element;}};// class template specialization:template <>class mycontainer <char> { char element; public: mycontainer (char arg) {element=arg;} char uppercase ();};// member of class template specialization:template <>char mycontainer<char>::uppercase(){ if ((element>='a')&&(element<='z')) element+='A'-'a'; return element;}int main () { mycontainer<int> myint (7); mycontainer<char> mychar ('j'); cout << myint.increase() << endl; cout << mychar.uppercase() << endl; return 0;} | 8J |
| template <> class mycontainer <char> { ... }; |
But more important than this prefix, is the <char> specialization parameter after the class template name. This specialization parameter itself identifies the type for which we are going to declare a template class specialization (char). Notice the differences between the generic class template and the specialization:
| template <class T> class mycontainer { ... };template <> class mycontainer <char> { ... }; |
When we declare specializations for a template class, we must also define all its members, even those exactly equal to the generic template class, because there is no "inheritance" of members from the generic template to the specialization.
Non-type parameters for templates
Besides the template arguments that are preceded by the class or typename keywords , which represent types, templates can also have regular typed parameters, similar to those found in functions. As an example, have a look at this class template that is used to contain sequences of elements:| // sequence template#include <iostream>using namespace std;template <class T, int N>class mysequence { T memblock [N]; public: void setmember (int x, T value); T getmember (int x);};template <class T, int N>void mysequence<T,N>::setmember (int x, T value) { memblock[x]=value;}template <class T, int N>T mysequence<T,N>::getmember (int x) { return memblock[x];}int main () { mysequence <int,5> myints; mysequence <double,5> myfloats; myints.setmember (0,100); myfloats.setmember (3,3.1416); cout << myints.getmember(0) << '\n'; cout << myfloats.getmember(3) << '\n'; return 0;} | 1003.1416 |
| template <class T=char, int N=10> class mysequence {..}; |
| mysequence<> myseq; |
| mysequence<char,10> myseq; |
Templates and multiple-file projects
From the point of view of the compiler, templates are not normal functions or classes. They are compiled on demand, meaning that the code of a template function is not compiled until an instantiation with specific template arguments is required. At that moment, when an instantiation is required, the compiler generates a function specifically for those arguments from the template.When projects grow it is usual to split the code of a program in different source code files. In these cases, the interface and implementation are generally separated. Taking a library of functions as example, the interface generally consists of declarations of the prototypes of all the functions that can be called. These are generally declared in a "header file" with a .h extension, and the implementation (the definition of these functions) is in an independent file with c++ code.
Because templates are compiled when required, this forces a restriction for multi-file projects: the implementation (definition) of a template class or function must be in the same file as its declaration. That means that we cannot separate the interface in a separate header file, and that we must include both interface and implementation in any file that uses the templates.
Since no code is generated until a template is instantiated when required, compilers are prepared to allow the inclusion more than once of the same template file with both declarations and definitions in a project without generating linkage errors.
| Namespaces | |
The format of namespaces is:
namespace identifier
{
entities
}
Where identifier is any valid identifier and entities is the set of classes, objects and functions that are included within the namespace. For example:
| namespace myNamespace{ int a, b;} |
| myNamespace::amyNamespace::b |
| // namespaces#include <iostream>using namespace std;namespace first{ int var = 5;}namespace second{ double var = 3.1416;}int main () { cout << first::var << endl; cout << second::var << endl; return 0;} | 53.1416 |
using
The keyword using is used to introduce a name from a namespace into the current declarative region. For example:| // using#include <iostream>using namespace std;namespace first{ int x = 5; int y = 10;}namespace second{ double x = 3.1416; double y = 2.7183;}int main () { using first::x; using second::y; cout << x << endl; cout << y << endl; cout << first::y << endl; cout << second::x << endl; return 0;} | 52.7183103.1416 |
The keyword using can also be used as a directive to introduce an entire namespace:
| // using#include <iostream>using namespace std;namespace first{ int x = 5; int y = 10;}namespace second{ double x = 3.1416; double y = 2.7183;}int main () { using namespace first; cout << x << endl; cout << y << endl; cout << second::x << endl; cout << second::y << endl; return 0;} | 5103.14162.7183 |
using and using namespace have validity only in the same block in which they are stated or in the entire code if they are used directly in the global scope. For example, if we had the intention to first use the objects of one namespace and then those of another one, we could do something like:
| // using namespace example#include <iostream>using namespace std;namespace first{ int x = 5;}namespace second{ double x = 3.1416;}int main () { { using namespace first; cout << x << endl; } { using namespace second; cout << x << endl; } return 0;} | 53.1416 |
Namespace alias
We can declare alternate names for existing namespaces according to the following format:namespace new_name = current_name;
Namespace std
All the files in the C++ standard library declare all of its entities within the std namespace. That is why we have generally included the using namespace std; statement in all programs that used any entity defined in iostream.| Exceptions | |
To catch exceptions we must place a portion of code under exception inspection. This is done by enclosing that portion of code in a try block. When an exceptional circumstance arises within that block, an exception is thrown that transfers the control to the exception handler. If no exception is thrown, the code continues normally and all handlers are ignored.
A exception is thrown by using the throw keyword from inside the try block. Exception handlers are declared with the keyword catch, which must be placed immediately after the try block:
| // exceptions#include <iostream>using namespace std;int main () { try { throw 20; } catch (int e) { cout << "An exception occurred. Exception Nr. " << e << endl; } return 0;} | An exception occurred. Exception Nr. 20 |
| throw 20; |
The exception handler is declared with the catch keyword. As you can see, it follows immediately the closing brace of the try block. The catch format is similar to a regular function that always has at least one parameter. The type of this parameter is very important, since the type of the argument passed by the throw expression is checked against it, and only in the case they match, the exception is catch.
We can chain multiple handlers (catch expressions), each one with a different parameter type. Only the handler that matches its type with the argument specified in the throw statement is executed.
If we use an ellipsis (...) as the parameter of catch, that handler will catch any exception no matter what the type of the throw exception is. This can be used as a default handler that catches all exceptions not caught by other handlers if it is specified at last:
| try { // code here}catch (int param) { cout << "int exception"; }catch (char param) { cout << "char exception"; }catch (...) { cout << "default exception"; } |
After an exception has been handled the program execution resumes after the try-catch block, not after the throw statement!.
It is also possible to nest try-catch blocks within more external try blocks. In these cases, we have the possibility that an internal catch block forwards the exception to its external level. This is done with the expression throw; with no arguments. For example:
| try { try { // code here } catch (int n) { throw; }}catch (...) { cout << "Exception occurred";} |
Exception specifications
When declaring a function we can limit the exception type it might directly or indirectly throw by appending a throw suffix to the function declaration:| float myfunction (char param) throw (int); |
If this throw specifier is left empty with no type, this means the function is not allowed to throw exceptions. Functions with no throw specifier (regular functions) are allowed to throw exceptions with any type:
| int myfunction (int param) throw(); // no exceptions allowedint myfunction (int param); // all exceptions allowed |
Standard exceptions
The C++ Standard library provides a base class specifically designed to declare objects to be thrown as exceptions. It is called exception and is defined in the <exception> header file under the namespace std. This class has the usual default and copy constructors, operators and destructors, plus an additional virtual member function called what that returns a null-terminated character sequence (char *) and that can be overwritten in derived classes to contain some sort of description of the exception.| // standard exceptions#include <iostream>#include <exception>using namespace std;class myexception: public exception{ virtual const char* what() const throw() { return "My exception happened"; }} myex;int main () { try { throw myex; } catch (exception& e) { cout << e.what() << endl; } return 0;} | My exception happened. |
All exceptions thrown by components of the C++ Standard library throw exceptions derived from this std::exception class. These are:
| exception | description |
| bad_alloc | thrown by new on allocation failure |
| bad_cast | thrown by dynamic_cast when fails with a referenced type |
| bad_exception | thrown when an exception type doesn't match any catch |
| bad_typeid | thrown by typeid |
| ios_base::failure | thrown by functions in the iostream library |
| try{ int * myarray= new int[1000];}catch (bad_alloc&){ cout << "Error allocating memory." << endl;} |
Because bad_alloc is derived from the standard base class exception, we can handle that same exception by catching references to the exception class:
| // bad_alloc standard exception#include <iostream>#include <exception>using namespace std;int main () { try { int* myarray= new int[1000]; } catch (exception& e) { cout << "Standard exception: " << e.what() << endl; } return 0;} | ||
| Type Casting | | |
Implicit conversion
Implicit conversions do not require any operator. They are automatically performed when a value is copied to a compatible type. For example:| short a=2000;int b;b=a; |
Implicit conversions also include constructor or operator conversions, which affect classes that include specific constructors or operator functions to perform conversions. For example:
| class A {};class B { public: B (A a) {} };A a;B b=a; |
Explicit conversion
C++ is a strong-typed language. Many conversions, specially those that imply a different interpretation of the value, require an explicit conversion. We have already seen two notations for explicit type conversion: functional and c-like casting:| short a=2000;int b;b = (int) a; // c-like cast notationb = int (a); // functional notation |
| // class type-casting#include <iostream>using namespace std;class CDummy { float i,j;};class CAddition { int x,y; public: CAddition (int a, int b) { x=a; y=b; } int result() { return x+y;}};int main () { CDummy d; CAddition * padd; padd = (CAddition*) &d; cout << padd->result(); return 0;} |
| padd = (CAddition*) &d; |
In order to control these types of conversions between classes, we have four specific casting operators: dynamic_cast, reinterpret_cast, static_cast and const_cast. Their format is to follow the new type enclosed between angle-brackets (<>) and immediately after, the expression to be converted between parentheses.
dynamic_cast <new_type> (expression)
reinterpret_cast <new_type> (expression)
static_cast <new_type> (expression)
const_cast <new_type> (expression)
The traditional type-casting equivalents to these expressions would be:
(new_type) expression
new_type (expression)
but each one with its own special characteristics:
dynamic_cast
dynamic_cast can be used only with pointers and references to objects. Its purpose is to ensure that the result of the type conversion is a valid complete object of the requested class.Therefore, dynamic_cast is always successful when we cast a class to one of its base classes:
| class CBase { };class CDerived: public CBase { };CBase b; CBase* pb;CDerived d; CDerived* pd;pb = dynamic_cast<CBase*>(&d); // ok: derived-to-basepd = dynamic_cast<CDerived*>(&b); // wrong: base-to-derived |
When a class is polymorphic, dynamic_cast performs a special checking during runtime to ensure that the expression yields a valid complete object of the requested class:
| // dynamic_cast#include <iostream>#include <exception>using namespace std;class CBase { virtual void dummy() {} };class CDerived: public CBase { int a; };int main () { try { CBase * pba = new CDerived; CBase * pbb = new CBase; CDerived * pd; pd = dynamic_cast<CDerived*>(pba); if (pd==0) cout << "Null pointer on first type-cast" << endl; pd = dynamic_cast<CDerived*>(pbb); if (pd==0) cout << "Null pointer on second type-cast" << endl; } catch (exception& e) {cout << "Exception: " << e.what();} return 0;} | Null pointer on second type-cast |
| Compatibility note: dynamic_cast requires the Run-Time Type Information (RTTI) to keep track of dynamic types. Some compilers support this feature as an option which is disabled by default. This must be enabled for runtime type checking using dynamic_cast to work properly. |
| CBase * pba = new CDerived;CBase * pbb = new CBase; |
When dynamic_cast cannot cast a pointer because it is not a complete object of the required class -as in the second conversion in the previous example- it returns a null pointer to indicate the failure. If dynamic_cast is used to convert to a reference type and the conversion is not possible, an exception of type bad_alloc is thrown instead.
dynamic_cast can also cast null pointers even between pointers to unrelated classes, and can also cast pointers of any type to void pointers (void*).
static_cast
static_cast can perform conversions between pointers to related classes, not only from the derived class to its base, but also from a base class to its derived. This ensures that at least the classes are compatible if the proper object is converted, but no safety check is performed during runtime to check if the object being converted is in fact a full object of the destination type. Therefore, it is up to the programmer to ensure that the conversion is safe. On the other side, the overhead of the type-safety checks of dynamic_cast is avoided.| class CBase {};class CDerived: public CBase {};CBase * a = new CBase;CDerived * b = static_cast<CDerived*>(a); |
static_cast can also be used to perform any other non-pointer conversion that could also be performed implicitly, like for example standard conversion between fundamental types:
| double d=3.14159265;int i = static_cast<int>(d); |
reinterpret_cast
reinterpret_cast converts any pointer type to any other pointer type, even of unrelated classes. The operation result is a simple binary copy of the value from one pointer to the other. All pointer conversions are allowed: neither the content pointed nor the pointer type itself is checked.It can also cast pointers to or from integer types. The format in which this integer value represents a pointer is platform-specific. The only guarantee is that a pointer cast to an integer type large enough to fully contain it, is granted to be able to be cast back to a valid pointer.
The conversions that can be performed by reinterpret_cast but not by static_cast have no specific uses in C++ are low-level operations, whose interpretation results in code which is generally system-specific, and thus non-portable. For example:
| class A {};class B {};A * a = new A;B * b = reinterpret_cast<B*>(a); |
const_cast
This type of casting manipulates the constness of an object, either to be set or to be removed. For example, in order to pass a const argument to a function that expects a non-constant parameter:| // const_cast#include <iostream>using namespace std;void print (char * str){ cout << str << endl;}int main () { const char * c = "sample text"; print ( const_cast<char *> (c) ); return 0;} | sample text |
typeid
typeid allows to check the type of an expression:typeid (expression)
This operator returns a reference to a constant object of type type_info that is defined in the standard header file <typeinfo>. This returned value can be compared with another one using operators == and != or can serve to obtain a null-terminated character sequence representing the data type or class name by using its name() member.
| // typeid#include <iostream>#include <typeinfo>using namespace std;int main () { int * a,b; a=0; b=0; if (typeid(a) != typeid(b)) { cout << "a and b are of different types:\n"; cout << "a is: " << typeid(a).name() << '\n'; cout << "b is: " << typeid(b).name() << '\n'; } return 0;} | a and b are of different types:a is: int *b is: int |
| // typeid, polymorphic class#include <iostream>#include <typeinfo>#include <exception>using namespace std;class CBase { virtual void f(){} };class CDerived : public CBase {};int main () { try { CBase* a = new CBase; CBase* b = new CDerived; cout << "a is: " << typeid(a).name() << '\n'; cout << "b is: " << typeid(b).name() << '\n'; cout << "*a is: " << typeid(*a).name() << '\n'; cout << "*b is: " << typeid(*b).name() << '\n'; } catch (exception& e) { cout << "Exception: " << e.what() << endl; } return 0;} | a is: class CBase *b is: class CBase **a is: class CBase*b is: class CDerived |
If the type typeid evaluates is a pointer preceded by the dereference operator (*), and this pointer has a null value, typeid throws a bad_typeid exception.
| Preprocessor directives | |
These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is considered to end. No semicolon (;) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash (\).
macro definitions (#define, #undef)
To define preprocessor macros we can use #define. Its format is:#define identifier replacement
When the preprocessor encounters this directive, it replaces any occurrence of identifier in the rest of the code by replacement. This replacement can be an expression, a statement, a block or simply anything. The preprocessor does not understand C++, it simply replaces any occurrence of identifier by replacement.
| #define TABLE_SIZE 100int table1[TABLE_SIZE];int table2[TABLE_SIZE]; |
| int table1[100];int table2[100]; |
| #define getmax(a,b) a>b?a:b |
| // function macro#include <iostream>using namespace std;#define getmax(a,b) ((a)>(b)?(a):(b))int main(){ int x=5, y; y= getmax(x,2); cout << y << endl; cout << getmax(7,x) << endl; return 0;} | 57 |
| #define TABLE_SIZE 100int table1[TABLE_SIZE];#undef TABLE_SIZE#define TABLE_SIZE 200int table2[TABLE_SIZE]; |
| int table1[100];int table2[200]; |
If the operator # is used before a parameter is used in the replacement sequence, that parameter is replaced by a string literal (as if it were enclosed between double quotes)
| #define str(x) #xcout << str(test); |
| cout << "test"; |
| #define glue(a,b) a ## bglue(c,out) << "test"; |
| cout << "test"; |
Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else and #elif)
These directives allow to include or discard part of the code of a program if a certain condition is met.#ifdef allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is. For example:
| #ifdef TABLE_SIZEint table[TABLE_SIZE];#endif |
#ifndef serves for the exact opposite: the code between #ifndef and #endif directives is only compiled if the specified identifier has not been previously defined. For example:
| #ifndef TABLE_SIZE#define TABLE_SIZE 100#endifint table[TABLE_SIZE]; |
The #if, #else and #elif (i.e., "else if") directives serve to specify some condition to be met in order for the portion of code they surround to be compiled. The condition that follows #if or #elif can only evaluate constant expressions, including macro expressions. For example:
| #if TABLE_SIZE>200#undef TABLE_SIZE#define TABLE_SIZE 200 #elif TABLE_SIZE<50#undef TABLE_SIZE#define TABLE_SIZE 50 #else#undef TABLE_SIZE#define TABLE_SIZE 100#endif int table[TABLE_SIZE]; |
The behavior of #ifdef and #ifndef can also be achieved by using the special operators defined and !defined respectively in any #if or #elif directive:
| #if !defined TABLE_SIZE#define TABLE_SIZE 100#elif defined ARRAY_SIZE#define TABLE_SIZE ARRAY_SIZEint table[TABLE_SIZE]; |
Line control (#line)
When we compile a program and some error happen during the compiling process, the compiler shows an error message with references to the name of the file where the error happened and a line number, so it is easier to find the code generating the error.The #line directive allows us to control both things, the line numbers within the code files as well as the file name that we want that appears when an error takes place. Its format is:
#line number "filename"
Where number is the new line number that will be assigned to the next code line. The line numbers of successive lines will be increased one by one from this point on.
"filename" is an optional parameter that allows to redefine the file name that will be shown. For example:
| #line 20 "assigning variable"int a?; |
Error directive (#error)
This directive aborts the compilation process when it is found, generating a compilation the error that can be specified as its parameter:| #ifndef __cplusplus#error A C++ compiler is required!#endif |
Source file inclusion (#include)
This directive has also been used assiduously in other sections of this tutorial. When the preprocessor finds an #include directive it replaces it by the entire content of the specified file. There are two ways to specify a file to be included:| #include "file"#include <file> |
If the file name is enclosed between angle-brackets <> the file is searched directly where the compiler is configured to look for the standard header files. Therefore, standard header files are usually included in angle-brackets, while other specific header files are included using quotes.
Pragma directive (#pragma)
This directive is used to specify diverse options to the compiler. These options are specific for the platform and the compiler you use. Consult the manual or the reference of your compiler for more information on the possible parameters that you can define with #pragma.If the compiler does not support a specific argument for #pragma, it is ignored - no error is generated.
Predefined macro names
The following macro names are defined at any time:| macro | value |
| __LINE__ | Integer value representing the current line in the source code file being compiled. |
| __FILE__ | A string literal containing the presumed name of the source file being compiled. |
| __DATE__ | A string literal in the form "Mmm dd yyyy" containing the date in which the compilation process began. |
| __TIME__ | A string literal in the form "hh:mm:ss" containing the time at which the compilation process began. |
| __cplusplus | An integer value. All C++ compilers have this constant defined to some value. If the compiler is fully compliant with the C++ standard its value is equal or greater than 199711L depending on the version of the standard they comply. |
| // standard macro names#include <iostream>using namespace std;int main(){ cout << "This is the line number " << __LINE__; cout << " of file " << __FILE__ << ".\n"; cout << "Its compilation began " << __DATE__; cout << " at " << __TIME__ << ".\n"; cout << "The compiler gives a __cplusplus value of " << __cplusplus; return 0;} | ||
| Input/Output with files | | |
- ofstream: Stream class to write on files
- ifstream: Stream class to read from files
- fstream: Stream class to both read and write from/to files.
| // basic file operations#include <iostream>#include <fstream>using namespace std;int main () { ofstream myfile; myfile.open ("example.txt"); myfile << "Writing this to a file.\n"; myfile.close(); return 0;} | [file example.txt]Writing this to a file |
But let's go step by step:
Open a file
The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream object (an instantiation of one of these classes, in the previous example this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.In order to open a file with a stream object we use its member function open():
open (filename, mode);
Where filename is a null-terminated character sequence of type const char * (the same type that string literals have) representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags:
| ios::in | Open for input operations. |
| ios::out | Open for output operations. |
| ios::binary | Open in binary mode. |
| ios::ate | Set the initial position at the end of the file. If this flag is not set to any value, the initial position is the beginning of the file. |
| ios::app | All output operations are performed at the end of the file, appending the content to the current content of the file. This flag can only be used in streams open for output-only operations. |
| ios::trunc | If the file opened for output operations already existed before, its previous content is deleted and replaced by the new one. |
| ofstream myfile;myfile.open ("example.bin", ios::out | ios::app | ios::binary); |
| class | default mode parameter |
| ofstream | ios::out |
| ifstream | ios::in |
| fstream | ios::in | ios::out |
The default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined.
File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters).
Since the first task that is performed on a file stream object is generally to open a file, these three classes include a constructor that automatically calls the open() member function and has the exact same parameters as this member. Therefor, we could also have declared the previous myfile object and conducted the same opening operation in our previous example by writing:
| ofstream myfile ("example.bin", ios::out | ios::app | ios::binary); |
To check if a file stream was successful opening a file, you can do it by calling to member is_open() with no arguments. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:
| if (myfile.is_open()) { /* ok, proceed with output */ } |
Closing a file
When we are finished with our input and output operations on a file we shall close it so that its resources become available again. In order to do that we have to call the stream's member function close(). This member function takes no parameters, and what it does is to flush the associated buffers and close the file:| myfile.close(); |
In case that an object is destructed while still associated with an open file, the destructor automatically calls the member function close().
Text files
Text file streams are those where we do not include the ios::binary flag in their opening mode. These files are designed to store text and thus all values that we input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value.Data output operations on text files are performed in the same way we operated with cout:
| // writing on a text file#include <iostream>#include <fstream>using namespace std;int main () { ofstream myfile ("example.txt"); if (myfile.is_open()) { myfile << "This is a line.\n"; myfile << "This is another line.\n"; myfile.close(); } else cout << "Unable to open file"; return 0;} | [file example.txt]This is a line.This is another line. |
| // reading a text file#include <iostream>#include <fstream>#include <string>using namespace std;int main () { string line; ifstream myfile ("example.txt"); if (myfile.is_open()) { while (! myfile.eof() ) { getline (myfile,line); cout << line << endl; } myfile.close(); } else cout << "Unable to open file"; return 0;} | This is a line.This is another line. |
Checking state flags
In addition to eof(), which checks if the end of file has been reached, other member functions exist to check the state of a stream (all of them return a bool value):bad()
Returns true if a reading or writing operation fails. For example in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left.
fail()
Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number.
eof()
Returns true if a file open for reading has reached the end.
good()
It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true.
In order to reset the state flags checked by any of these member functions we have just seen we can use the member function clear(), which takes no parameters.
get and put stream pointers
All i/o streams objects have, at least, one internal stream pointer:ifstream, like istream, has a pointer known as the get pointer that points to the element to be read in the next input operation.
ofstream, like ostream, has a pointer known as the put pointer that points to the location where the next element has to be written.
Finally, fstream, inherits both, the get and the put pointers, from iostream (which is itself derived from both istream and ostream).
These internal stream pointers that point to the reading or writing locations within a stream can be manipulated using the following member functions: tellg() and tellp() These two member functions have no parameters and return a value of the member type pos_type, which is an integer data type representing the current position of the get stream pointer (in the case of tellg) or the put stream pointer (in the case of tellp). seekg() and seekp() These functions allow us to change the position of the get and put stream pointers. Both functions are overloaded with two different prototypes. The first prototype is:
seekg ( position );
seekp ( position );
Using this prototype the stream pointer is changed to the absolute position position (counting from the beginning of the file). The type for this parameter is the same as the one returned by functions tellg and tellp: the member type pos_type, which is an integer value.
The other prototype for these functions is:
seekg ( offset, direction );
seekp ( offset, direction );
Using this prototype, the position of the get or put pointer is set to an offset value relative to some specific point determined by the parameter direction. offset is of the member type off_type, which is also an integer type. And direction is of type seekdir, which is an enumerated type (enum) that determines the point from where offset is counted from, and that can take any of the following values:
| ios::beg | offset counted from the beginning of the stream |
| ios::cur | offset counted from the current position of the stream pointer |
| ios::end | offset counted from the end of the stream |
| // obtaining file size#include <iostream>#include <fstream>using namespace std;int main () { long begin,end; ifstream myfile ("example.txt"); begin = myfile.tellg(); myfile.seekg (0, ios::end); end = myfile.tellg(); myfile.close(); cout << "size is: " << (end-begin) << " bytes.\n"; return 0;} | size is: 40 bytes. |
Binary files
In binary files, to input and output data with the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data, and data may not use the separation codes used by text files to separate elements (like space, newline, etc...).File streams include two member functions specifically designed to input and output binary data sequentially: write and read. The first one (write) is a member function of ostream inherited by ofstream. And read is a member function of istream that is inherited by ifstream. Objects of class fstream have both members. Their prototypes are:
write ( memory_block, size );
read ( memory_block, size );
Where memory_block is of type "pointer to char" (char*), and represents the address of an array of bytes where the read data elements are stored or from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read or written from/to the memory block.
| // reading a complete binary file#include <iostream>#include <fstream>using namespace std;ifstream::pos_type size;char * memblock;int main () { ifstream file ("example.txt", ios::in|ios::binary|ios::ate); if (file.is_open()) { size = file.tellg(); memblock = new char [size]; file.seekg (0, ios::beg); file.read (memblock, size); file.close(); cout << "the complete file content is in memory"; delete[] memblock; } else cout << "Unable to open file"; return 0;} | the complete file content is in memory |
First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at the end of the file. This way, when we call to member tellg(), we will directly obtain the size of the file. Notice the type we have used to declare variable size:
| ifstream::pos_type size; |
| int size;size = (int) file.tellg(); |
| memblock = new char[size]; |
| file.seekg (0, ios::beg);file.read (memblock, size);file.close(); |
Buffers and Synchronization
When we operate with file streams, these are associated to an internal buffer of type streambuf. This buffer is a memory block that acts as an intermediary between the stream and the physical file. For example, with an ofstream, each time the member function put (which writes a single character) is called, the character is not written directly to the physical file with which the stream is associated. Instead of that, the character is inserted in that stream's intermediate buffer.When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream) or simply freed (if it is an input stream). This process is called synchronization and takes place under any of the following circumstances:
- When the file is closed: before closing a file all buffers that have not yet been flushed are synchronized and all pending data is written or read to the physical medium.
- When the buffer is full: Buffers have a certain size. When the buffer is full it is automatically synchronized.
- Explicitly, with manipulators: When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: flush and endl.
- Explicitly, with member function sync(): Calling stream's member function sync(), which takes no parameters, causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it returns 0.
