Do Default Parameters in Virtual Functions Inherit to Derived Classes?

Can Virtual Functions Have Default Parameters?

Problem:

When declaring a virtual function with default parameters in a base class, do derived classes inherit these defaults?

Answer:

No, defaults in virtual functions are not inherited by derived classes. The default used is determined by the static type of the object when the function is called.

Explanation:

The C Standards (C 03 and C 11) specify that virtual function calls use the default arguments declared in the function definition determined by the static type of the pointer or reference used to invoke the function.

Example:

Consider the following code:

struct Base {
  virtual void f(int a = 7);
};
struct Der : public Base {
  void f(int a);
};

When calling f() through a pointer to a Base object, the default 7 will be used:

Base* pb = new Base;
pb->f(); // uses the default 7

However, when calling f() through a pointer to a Der object, the derived class's default will not be used:

Der* pd = new Der;
pd->f(); // error: no default argument for this function

Practice and Compiler Considerations:

While the C Standards dictate the behavior, some compilers may implement virtual function default parameters differently. However, it is recommended to follow the Standard's guidelines to ensure consistent behavior across compilers.

Code Demonstration:

The following code demonstrates the default parameter behavior:

struct Base { virtual string Speak(int n = 42); };
struct Der : public Base { string Speak(int n = 84); };

int main()
{
    Base b1;
    Der d1;

    Base *pb1 = &b1, *pb2 = &d1;
    Der *pd1 = &d1;
    cout << pb1->Speak() << "\n" // Base 42
         << pb2->Speak() << "\n" // Der 42
         << pd1->Speak() << "\n" // Der 84
         << endl;
}

Output:

Base 42
Der 42
Der 84

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