Default arguments and function overloading

Default arguments

A default argument is a value given in the function declaration that is used automatically when the caller does not supply that argument. Default values are given from the right side: once a parameter has a default, all parameters to its right must also have defaults.

float simpleInterest(float p, float t, float r = 5.0);   // r defaults to 5

simpleInterest(1000, 2);        // r = 5.0 is used
simpleInterest(1000, 2, 8);     // r = 8, default ignored

Advantages: fewer arguments to write, one function covers common cases, and existing calls keep working when a new parameter is added.

Default arguments and function overloading

A function with default arguments behaves like a set of overloaded functions. The same effect is obtained by writing several functions with the same name and different parameter lists.

#include <iostream>
using namespace std;

// Version 1: using default arguments
int volume1(int l, int w = 1, int h = 1) {
    return l * w * h;
}

// Version 2: using function overloading (same behaviour)
int volume2(int l)               { return l; }
int volume2(int l, int w)        { return l * w; }
int volume2(int l, int w, int h) { return l * w * h; }

int main() {
    cout << volume1(5) << " " << volume1(5, 4) << " " << volume1(5, 4, 3) << endl;
    cout << volume2(5) << " " << volume2(5, 4) << " " << volume2(5, 4, 3) << endl;
    return 0;
}

Output

5 20 60
5 20 60

The single function volume1 replaces the three overloaded functions of volume2.

Caution (ambiguity): if overloaded functions and default arguments are mixed carelessly, calls can become ambiguous. For example, with void f(int a, int b = 0); and void f(int a);, the call f(5) matches both and the compiler reports an error.