Structure of C++ Program

The program written in C++ language follows this basic structure. The sequence of sections should be as they are in the basic structure. A C-program should have one or more sections but the sequence of sections is to be followed.

  1. Documentation section
  2. Linking section
  3. Definition section
  4. Global declaration section & class declarations
  5. Member function definition
  6. Main function
    section main()
    Declaration section
    Executable section


1. // my first program in C++           //Documentation Section       
2. #include<iostream>                   //Linking Section 
3. using namespace std; 
4. #define MAX 25                       //Definition Section          
5. int main ()                          //main section 
6. {
7.   cout << "Hello World!";
8.   return 0;
9. }

Output:-Hello World!
  1. // 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.
  2. #include<iostream> : Lines beginning with a hash 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 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.
  3. 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.
  4. #define MAX 25 :It is used to declare some constants and assign them some value. e.g. #define MAX 25 Here #define is a compiler directive which tells the compiler whenever MAX is found in the program replace it with 25.
  5. 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.
  6. 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.
  7. 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.
    • Note 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).
  8. 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++ console program.

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.We could have written:

int main ()  {   cout << "Hello World!";   return 0;   }

All in just one line and this would have had exactly the same meaning as the previous code. 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.

Example 2

// my second program in C++
#include <iostream>
using namespace std;
#define MAX 25
int main ()
    cout << "Hello World! ";
    cout << "I'm a C++ program  ";
    return 0;
Hello World! I'm a C++ program Max=25

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