Default Argument

A function parameter can be given a default value which is used when the caller does not pass an argument for it:

void greet(std::string name = "world") {
  std::cout << "Hello " << name << "\n";
}

greet();         // Hello world
greet("there");  // Hello there

Parameters with default arguments can only appear at the end of a parameter list:

void f(int a, int b = 2, int c = 3);  // OK.
void g(int a = 1, int b);             // ERROR.

Default arguments are always filled from the end of the parameter list, so a caller cannot skip an earlier default argument:

void f(int a, int b = 2, int c = 3);
f(1);        // a = 1, b = 2, c = 3
f(1, 5);     // a = 1, b = 5, c = 3
f(1, 5, 9);  // a = 1, b = 5, c = 9

Default values must go in the function declaration and not the definition:

void f(int a, int b = 2);  // Declaration.
void f(int a, int b) {}    // Definition.