Default arguments and their syntax

A default argument is a value given to a function parameter in the function declaration. If the caller leaves that argument out, the compiler automatically uses the default value. If the caller supplies a value, the default is ignored.

Syntax

return_type function_name(type p1, type p2 = default_value, type p3 = default_value);

• Default values are given from right to left: once a parameter has a default, all parameters to its right must also have defaults.
• The default is written once, in the declaration (prototype) or in the definition if there is no prototype, not in both.
• Default values may be constants, global variables or expressions, but not local variables.

Example

#include <iostream>
using namespace std;

// w and h take default values
float volume(float l, float w = 1, float h = 1)
{
    return l * w * h;
}

int main()
{
    cout << volume(5) << endl;         // 5*1*1 = 5
    cout << volume(5, 4) << endl;      // 5*4*1 = 20
    cout << volume(5, 4, 3) << endl;   // 5*4*3 = 60
    return 0;
}

Valid: void f(int a, int b = 2, int c = 3); Invalid: void f(int a = 1, int b, int c = 3); because b has no default but lies to the right of a defaulted parameter.

Advantage: one function can be called in several ways, which reduces the number of overloaded functions we must write.