print address of virtual member function

Question

I am trying to print the address of a virtual member function. If I only wants to print the address of the function I can write:

print("address: %p", &A::func);

But I want to do something like this:

A *b = new B();

printf("address: %p", &b->func); 
printf("address: %p", &b->A::func);

however this does not compile, is it possible to do something like this even do looking up the address in the vtable is done in runtime?

Answer 1

First, you could be a bit more specific on what your compiler fails. Then you should have a look into pointer to member syntax, this is a bit tricky. Googling for that immediately found me that: http://www.goingware.com/tips/member-pointers.html

Also this is definitively not a C question.

Answer 2

Doesn't make a lot a of sense to me. If you have a normal function:

void f( int n ) {
}

then you can take its address:

f

but you cannot take the address of a function call, which is what you seem to want to do.

Answer 3

From what I can tell in the standard, the only time you get dynamic binding is during a virtual function call. And once you've called a function, you're executing the statements within the function (i.e., you can't "stop halfway" into the call and get the address.)

I think it's impossible.

Answer 4

Pointers to member functions are not always simple memory addresses. See the table in this article showing the sizes of member function pointers on different compilers - some go up to 20 bytes.

As the article outlines a member function pointer is actually a blob of implementation-defined data to help resolve a call through the pointer. You can store and call them OK, but if you want to print them, what do you print? Best to treat it as a sequence of bytes and get its length via sizeof.

Answer 5

Will this code print the right address? It will print the address of the function in the vtable, but is it correct:)?

struct A
{
    virtual void func() { cout << "A::func" << endl; }
};

struct B : public A
{
    void func() { cout << "B::func" << endl; }
};

typedef void (*functionPtr)();

int main()
{
    A *b = new B();

    int *vptr = *(int**)&b;
    int *vtable = (int *)*vptr; 

    functionPtr fp = (functionPtr)vtable[0];

    printf("address A::func = %p\n", fp);

    return 0;
}