Pointer-to-member-function performs virtual dispatch?

Question

I executed the following code.

#include <iostream>

class Base
{
public:
    virtual void func()
    {
        std::cout<<"Base func called"<<std::endl;
    }
};

class Derived: public Base
{
public:
    virtual void func()  override
    {
        std::cout<<"Derived func called"<<std::endl;
    }
};

int main()
{
    void (Base::*func_ptr)()=&Base::func; //Yes, the syntax is very beautiful.
    Base* bptr=new Derived();
    (bptr->*func_ptr)();
}

My expected output was Base func called. However, Instead, the output was

Derived func called

Which surprised me, because I think that func_ptr should be able to see only Base members(because I thought that func_ptr doesn't access the member function via _vptr, but the function address itself.

I would like to know, how the virtual dispatch takes place in this case(how the access to virtual table takes place), and where this behavior is defined in C++ standard(I couldn't find anything)?

Answer 1

Refer to [expr.call], specifically here

[If the selected function is virtual], its final overrider in the dynamic type of the object expression is called; such a call is referred to as a virtual function call

Whether you call the function through a pointer or by class member access is the same (ref); the actual function called for a virtual function ultimately depends on the actual type of the object it is being called on.

A few (non-normative) notes in the standard under [class.virtual] say much the same thing:

[Note 3: The interpretation of the call of a virtual function depends on the type of the object for which it is called (the dynamic type)

[Note 4: [...] a virtual function call relies on a specific object for determining which function to invoke.

---

If you would like to know how the virtual function dispatch takes place, you will need to seek out a specific implementation, because the how is not standardized.

(I really enjoyed the article A basic glance at the virtual table, which shows one possible way you could implement it using C)