C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 33:
[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.