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

C++ FAQ / Section 33 / FAQ 33.11

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?

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

No!

int f(char x, float y);
int g(char x, float y);

typedef int(*FunctPtr)(char,float);

int callit(FunctPtr p, char x, float y)
{
  return p(x, y);
}

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

Technical details: void* pointers are pointers to data, and function pointers point to functions. The language does not require functions and data to be in the same address space, so, by way of example and not limitation, on architectures that have them in different address spaces, the two different pointer types will not be comparable.

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.