Print address of virtual member function

c++ virtual vtable
hidayat picture hidayat · Jun 18, 2010 · Viewed 13.2k times · Source

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

Lukas W picture Lukas W · Aug 30, 2012

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.

Related questions

undefined reference to vtable - virtual member, classes generated by gsoap

gsoap with its tools wsdl2h and soapcpp2 provided me with a soapStub.h file containing the following: class SOAP_CMAC ns2__SOAPKunden { public: std::string *adresszusatz; // ... public: virtual int soap_type() const { return 7; } // ... ns2__SOAPKunden() : adresszusatz(NULL), x(NULL) { } // …

Read More
Number of Virtual tables and Virtual Pointers in a C++ Program

Let say we have below program: class A { public: virtual fun(){}; }; class B:public A { public: virtual fun(){}; }; int main() { A a1; B b1; } My question is how many vtables and how many vptrs will be created, when we run …

Read More
Virtual/pure virtual explained

What exactly does it mean if a function is defined as virtual and is that the same as pure virtual?

Read More