Virtual functions default parameters

Question

Could anybody explain where c++ compilers keep default values for parameters for virtual functions? I know it is a bad idea to change these parameters in child classes but why? Thanks.

Answer 1

It's a bad idea because they aren't kept anywhere.

The default values that are used will be those defined in the static (compile-time) type. So if you were to change the default parameters in an override, but you called the function through a base class pointer or reference, the default values in the base would be used.

#include <iostream>  struct Base {     virtual ~Base(){ }     virtual void foo(int a=0) { std::cout << "base: " << a << std::endl; } };  struct Derived : public Base {     virtual ~Derived() { }     virtual void foo(int a=1) { std::cout << "derived: " << a << std::endl; } };  int main() {     Base* derived = new Derived();     derived->foo();    // prints "derived: 0"     delete derived; }