Print address of virtual member function

Question

I am trying to print the address of a virtual member function. If I know which class implements 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, perhaps looking up the address in the vtable at runtime?

Answer 1

Currently there is no standard way of doing this in C++ although the information must be available somewhere. Otherwise, how could the program call the function? However, GCC provides an extension that allows us to retrieve the address of a virtual function:

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

...assuming the member function has the prototype void func(). This can be pretty useful when you want to cache the address of a virtual function or use it in generated code. GCC will warn you about this construct unless you specify -Wno-pmf-conversions. It's unlikely that it works with any other compiler.

Answer 2

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.