当我使用根据基类定义的成员函数指针时，编译器如何调用派生成员函数？

Question

The code below works, but I'm not quite sure I understand why the member function pointer memfunc_ptr ends up pointing to the correct function Derived::member_func() (see example here ). I know that a member function pointer defines an offset into the class of the object, for which it's defined, in this case class Base . So, to justify the result obtained by the code, ie, that the member function Derived::member_func is called, instead of Base::member_func , I have to conclude that this offset is applied to the vtable of the class Derived , as member_func() is virtual, and the object d is of class Derived . Does that make sense?

#include <iostream>
class Base {
public: 
    virtual void member_func() {
    std::cout << "Base" << '\n'; };
};

class Derived : public Base {
public:
    virtual void member_func() { std::cout << "Derived" << '\n'; };
};

int main() {
    typedef void (Base::*MFP)();
    MFP memfunc_ptr;
    memfunc_ptr = &Base::member_func;
    Derived d;
    (d.*memfunc_ptr)(); 
}

Answer 1

member_func() is a virtual function. The compiler will hence always call the most appropriate function for the real type of the object that is pointed to. The way this is ensured is implementation specific .

The most frequently way to do it is to use a vtable , a table of funtion pointers. A pointer to the vtable is initialised during the construction of the object. Every time you refer to such a virtual function, the compiler will generate code to find the function pointer in the vtable (using an ofset in the vtable). And when you use a pointer to a virtual function, the compiler will generate code to find the right pointer in the vtable.

This article will show you how this work with more details.