A function is the fundamental building block of any program, as functions perform the tasks required in that program. The most important in a C++ program is called main ( ). The main ( ) function is automatically executed when the program is run. In effect, main ( ) is the program. A typical main function looks like this:

Int main (void)

{

// Your code here

return 0;

}

• The // characters mark the beginning of a comment all text following // on a line is ignored by the compiler.

• The first line specifies the return type of the function, its name, and what arguments, if any the function requires.

• In the above example the function is called main, and will return a value of type int (thus int main). This function requires no arguments, or inputs to perform its task, so the type is specified as void.

• The instructions to be executed are enclosed between the curly brackets.

• The return keyword is used to exit the function and pass a return value back to the functions caller. If return is omitted, a function call will exit at the closing curly bracket with no value returned. However, in this example, the compiler would not allow such as an implied return because the main function was defined as returning an int value. Thus, in this function a value must explicitly be returned.
Where a function does not need to return a value, it may be defined as void, so for a function which uses two integer values, but returns no result:

Void NoValueReturned(int x, int y)

  {

  // code here

  }

Writing functions

Consider a function, which calculates the square of a number:

Int Square(int input)

{

return(input * input);

}

This function returns a value of type int, is called Square and requires one input argument, of type int, called input by that function. Inside the function, input refers to whatever int value was passed to the function. As can be seen below, the calling code does not have to call the int it passes to this function input :so long as the callers value is of type int it can be used as an input to Square(). Similarly to a variable, before this function can be used by other code, it must first be declared:

void main (void)

{

int Square( int ); //declare Square ( )

int x, y, result; //declare variables

x = 4 * 2;

y = x / 7;

result = Square ( y ); //call square function with y as input

}