Default Function Arguments

A default argument is a value provided in a function declaration that is automatically assigned by the compiler if the caller of the function doesn't provide a value for the argument when the function is called. For example:

#include <iostream>

using std::cout;
using std::endl;

void rectangle_area(int = 3, int = 5);       // Default value for width is 3, default value for height is 5

int main()
{
    cout << rectangle_area() << endl;        // Calls rectangle_area(3, 5), so this prints 15.
   
    cout << rectangle_area(6) << endl;       // Calls rectangle_area(6, 5), so this prints 30.
   
    cout << rectangle_area(2, 6) << endl;    // Neither default value used, so this prints 12.

    return 0;
}

int rectangle_area(int width, int height)
{
    return width * height;
}

Default Arguments and Constructors

Default arguments are often used with constructors. A constructor with default values for all of its arguments can serve as a default constructor. For example:

class Date
{
private:

    int month;
    int day;
    int year;
    
public:

    Date(int = 1, int = 1, int = 1900);     // Serves as a default constructor.
    ...
};

Date::Date(int month, int day, int year)
{
    this->month = month;
    this->day = day;
    this->year = year;
}

...

Since the Date constructor has default values for all three of its arguments, there is no need to write a separate default constructor:

int main()
{
    Date d1;                  // Date object initialized to 1/1/1900.
    Date d2(10);              // Date object initialized to 10/1/1900.
    Date d3(4, 23);           // Date object initialized to 4/23/1900.
    Date d4(12, 7, 1941);     // Date object initialized to 12/7/1941.
    
    ...
    
    return 0;
}

Restrictions on Default Arguments

If not all of a function's arguments have default values, the ones that do typically must be the trailing arguments in the argument list.

void f(int, int = 2, int = 3);     // Default values are for trailing arguments, so this is okay.
void g(int = 1, int = 2, int);     // Error: default values supplied for first two arguments.
void h(int, int = 3, int);         // Error: default value supplied only for middle argument.

(There are some exceptions to this rule, particularly when it comes to more advanced template functions.)