Functions in c++

  • A function is a subroutine that contains one or more C++ statements and performs a specific task.
  • Every program that you have written so far has used one function: main( ).
  • They are called the building blocks of C++ because a program is a collection of functions.
  • All of the “action” statements of a program are found within functions.
  • Thus, a function contains the statements that you typically think of as being the executable part of a program.

Function declaration

  • A function has a name and a type, much like a variable. The function’s type is defined by its return value, that is, the value the function passes back to the program.
  • In addition, the type of arguments required by a function is important. When a function is declared, the compiler must therefore be provided with information on
    • the name and type of the function and
    • the type of each argument.
    Example:
     int toupper(int);
double pow(double, double);
  • This informs the compiler that the function toupper() is of type int, i.e. its return value is of type int, and it expects an argument of type int.
  • The second function pow() is of type double and two arguments of type double must be passed to the function when it is called.
  • The types of the arguments may be followed by names, however, the names are viewed as a comment only.

Function Definition

  • The definition consists of a line called the declarator, followed by the function body.
  • The function body is composed of the statements that make up the function, delimited by braces.
    • void starline() //declarator
  {
   for(int j=0; j<45; j++) //function body
     cout << ‘*’;
   cout << endl;
  }

Passing Pointers to Functions

  • To pass a pointer as an argument, you must declare the parameter as a pointer type.
    •  // Pass a pointer to a function.
#include <iostream>
using namespace std;

void f(int *j);

int main()
{
   int i;
   f(&i);
   cout << i;
   return 0;
}

void f(int *j)
{
  *j =100; //var pointed to by j is assigned 100
}

Passing an Array

  • When an array is an argument to a function, the address of the first element of the array is passed, not a copy of the entire array. (Recall that an array name without any index is a pointer to the first element in the array.)
  • This means that the parameter declaration must be of a compatible type.
    • //Pass a pointer to a function.

#include <iostream>
using namespace std;

void display(int num[10]);

int main()
{
  int t[10], t;
  for(i=0; i<10; ++t)
    t[i]=i;
  display(t); // pass array t to a function
  return 0;

//Print some number.
void display(int num[10]) // Parameter declared as a sized array
{
  int i;
  for(i=0; i <10; i++)
  cout << num[i] << ' ';
}