c++ - Why can I access a derived private member function via a base class pointer to a derived object?

Question

#include<iostream>

using namespace std;
class base
{
public:
    virtual void add() {
        cout << "hi";
    }
};

class derived : public base
{
private:
    void add() {
        cout << "bye";
    }
};

int main()
{
    base *ptr;
    ptr = new derived;
    ptr->add();
    return 0;
}

Output is bye

I dont have a problem with how this is implemented. I understand you use vtables and the vtable of derived contains the address of the new add() function. But add() is private shouldn't compiler generate an error when I try to access it outside the class? Somehow it doesn't seem right.

Answer 1

add() is only private in derived, but the static type you have is base* - thus the access restrictions of base apply.
In general you can't even know at compile time what the dynamic type of a pointer to base will be, it could e.g. change based on user input.

This is per C++03 §11.6:

The access rules (clause 11) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it.
[...] Access is checked at the call point using the type of the expression used to denote the object for which the member function is called [...]. The access of the member function in the class in which it was defined [...] is in general not known.