Can I convert a pointer-to-member-function to a void*?, C++ FAQ

C++ FAQ / Section 33 / FAQ 33.10

Section 33:

33.1	Is the type of "pointer-to-member-function" different from "pointer-to-function"?
33.2	How do I pass a pointer-to-member-function to a signal handler, X event callback, system call that starts a thread/task, etc?
33.3	Why do I keep getting compile errors (type mismatch) when I try to use a member function as an interrupt service routine?
33.4	Why am I having trouble taking the address of a C++ function?
33.5	How can I avoid syntax errors when creating pointers to members?
33.6	How can I avoid syntax errors when calling a member function using a pointer-to-member-function?
33.7	How do I create and use an array of pointer-to-member-function?
33.8	How do I declare a pointer-to-member-function that points to a `const` member function?
33.9	What is the difference between the `.` and `->` operators?
33.10	Can I convert a pointer-to-member-function to a `void*`?
33.11	Can I convert a pointer-to-function to a `void*`?
33.12	I need something like function-pointers, but with more flexibility and/or thread-safety; is there another way?
33.13	What the heck is a functionoid, and why would I use one?
33.14	Can you make functionoids faster than normal function calls?
33.15	What's the difference between a functionoid and a functor?

[33.10] Can I convert a pointer-to-member-function to a void*?

No!

class Fred {
public:
  int f(char x, float y);
  int g(char x, float y);
  int h(char x, float y);
  int i(char x, float y);
  ...
};

// FredMemFn points to a member of Fred that takes (char,float)
typedef  int (Fred::*FredMemFn)(char x, float y);

#define CALL_MEMBER_FN(object,ptrToMember)  ((object).*(ptrToMember))

int callit(Fred& o, FredMemFn p, char x, float y)
{
  return CALL_MEMBER_FN(o,p)(x, y);
}

int main()
{
  FredMemFn p = &Fred::f;
  void* p2 = (void*)p;                  // ← illegal!!
  Fred o;
  callit(o, p, 'x', 3.14f);             // okay
  callit(o, FredMemFn(p2), 'x', 3.14f); // might fail!!
  ...
}

Technical details: pointers to member functions and pointers to data are not necessarily represented in the same way. A pointer to a member function might be a data structure rather than a single pointer. Think about it: if it's pointing at a virtual function, it might not actually be pointing at a statically resolvable pile of code, so it might not even be a normal address — it might be a different data structure of some sort.

Please do not email me if the above seems to work on your particular version of your particular compiler on your particular operating system. I don't care. It's illegal, period.