C++ Pointer to virtual function

c++ function-pointers virtual-functions member-function-pointers
Chris picture Chris · Jul 20, 2011 · Viewed 20.5k times · Source

If you have a struct like this one

struct A {
    void func();
};

and a reference like this one

A& a;

you can get a pointer to its func method like this:

someMethod(&A::func);

Now what if that method is virtual and you don't know what it is at run-time? Why can't you get a pointer like this?

someMethod(&a.func);

Is it possible to get a pointer to that method?

Answer

Armen Tsirunyan picture Armen Tsirunyan · Jul 20, 2011

Pointers to members take into account the virtuality of the functions they point at. For example:

#include <iostream>
struct Base
{
    virtual void f() { std::cout << "Base::f()" << std::endl; }
};

struct Derived:Base
{
    virtual void f() { std::cout << "Derived::f()" << std::endl; }
};


void SomeMethod(Base& object, void (Base::*ptr)())
{
    (object.*ptr)();    
}


int main()
{
    Base b;
    Derived d;
    Base* p = &b;
    SomeMethod(*p, &Base::f); //calls Base::f()
    p = &d;
    SomeMethod(*p, &Base::f); //calls Derived::f()    
}

Outputs:

Base::f()
Derived::f()

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