Default values in parameters

In C++, functions can also have optional parameters, for which no arguments are required in the call, in such a way that, for example, a function with three parameters may be called with only two. For this, the function shall include a default value for its last parameter, which is used by the function when called with fewer arguments.

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) << '\n';
  cout << divide (20,4) << '\n';
  return 0;
}

Output:

6
5

In this example, there are two calls to function divide. In the first one:

divide (12)

The call only passes one argument to the function, even though the function has two parameters. In this case, the function assumes the second parameter to be 2 (notice the function definition, which declares its second parameter as int b=2). Therefore, the result is 6.

In the second call:

divide (20,4)

The call passes two arguments to the function. Therefore, the default value for b (int b=2) is ignored, and b takes the value passed as argument, that is 4, yielding a result of 5.