C++ provides rich collection of functions ready to be used for various tasks. The tasks to be performed by each of these are already written, debugged and compiled, their definitions alone are grouped and tored in files called header files. Such ready-to-use sub programs are called pre-defined functions Of built-in functions or Library Functions.

The user can also define new functions to perform a specific task. These are called as user-defined functions. User-defined functions are created by the user. A function can optionally define input parameters that enable callers to pass arguments into the function. A function can also optionally return a value as output. Functions are useful for encapsulating common operations in a single reusable block, ideally with a name that clearly describes what the function does

#include < iostream >
using namespace std;
double area(const double r,const double pi=3.14)
{
return(pi*r*r);
}
int main 0
{
double rad,res;
cout << "\nEnter Radius :";
cin >> rad; .
res=area(rad);
cout << "\nThe Area of Circle =" << res;
return 0;
}
Output:
Enter Radius :5
The Area of Circle =78.5

(i) The Return_Data _Type is any valid data type of C++
(ii) The Function_name is a user-defined identifier. 
(iii) The parameter list, which is optional, is a list of parameters, i.e., a list of variables preceded by data types and separated by commas.
(iv) The body of the function comprises C++ statements that are required to perform the intended task of this function.