Virtual member functions and std::tr1::function: How does this work?

Question

Here is a sample piece of code. Note that B is a subclass of A and both provide a unique print routine. Also notice in main that both bind calls are to &A::print, though in the latter case a reference to B is passed.

#include <iostream>
#include <tr1/functional>

struct A
{
    virtual void print()
    {
        std::cerr << "A" << std::endl;
    }
};

struct B : public A
{
    virtual void print()
    {
        std::cerr << "B" << std::endl;
    }
};

int main (int argc, char * const argv[])
{
    typedef std::tr1::function<void ()> proc_t;

    A a;
    B b;

    proc_t a_print = std::tr1::bind(&A::print, std::tr1::ref(a));
    proc_t b_print = std::tr1::bind(&A::print, std::tr1::ref(b));

    a_print();
    b_print();

    return 0;
}

Here is the output I see compiling with GCC 4.2:

A
B

I would consider this correct behavior, but I am at a loss to explain how it is working properly given that the std::tr1::functions were bound to &A::print in both cases. Can someone please enlighten me?

EDIT: Thanks for the answers. I am familiar with inheritance and polymorphic types. What I am interested in is what does &A::print mean? Is it an offset into a vtable, and that vtable changes based on the referred object (in this case, a or b?) From a more nuts-and-bolts perspective, how does this code behave correctly?

Answer 1

Because print() is declared virtual, A is a polymorphic class. By binding to the print function pointer, you will be calling through an A pointer, much in the same way as:

A* ab = &b;
ab->print();

In the ->print call above, you would expect polymorphic behavior. Same it true in your code as well. And this is a Good Thing, if you ask me. At least, most of the time. :)

Answer 2

This works in the same manner as it would have worked with plain member function pointers. The following produces the same output:

int main ()
{
    A a;
    B b;
    typedef void (A::*fp)();
    fp p = &A::print;
    (a.*p)(); // prints A
    (b.*p)(); // prints B
}

It would have been surprising if boost/tr1/std::function did anything different since they presumably store these pointers to member functions under the hood. Oh, and of course no mention of these pointers is complete without a link to the Fast Delegates article.