Default Arguments

C++ has added the ability to have default arguments be defined when you declare your functions. To define a default value, you must specify the parameter name, and then assign it a value.

double circleArea(double radius = 20)
{
  const double PI = 3.1415;
  return PI * radius * radius;
}

If someone calls this function without passing an argument to the function, the computer will use 20 as the parameter. If they pass a argument , then that argument will be used as the parameter.

double area1 = circleArea();
double area2 = circleArea(33.2);

In some ways, this is like function overloading, but simpler to use. For example, both of the following examples are equivalent:

// overloaded example
double circleArea()
{
  return circleArea(20);
}

double circleArea(double radius)
{
  const double PI = 3.1415;
  return PI * radius * radius;
}

// default value value example
double circleArea(double radius = 20)
{
  const double PI = 3.1415;
  return PI * radius * radius;
}

When calling a function with a default value, once a default value is used, then the remaining parameters must use their default value.

double boxVolume(double height = 10, double width = 10, double depth = 10) 
{
  return height * width * depth;
}

cout << boxVolume() << endl; // valid call
cout << boxVolume(10,20,30) << endl; // valid call
cout << boxVolume(10,20) << endl; // valid call

/// errors at this point
cout << boxVolume(,,10) << endl; // ivalid call
cout << boxVolume(10, ,20) << endl; // ivalid call

Default Arguments was originally found on Access 2 Learn