About sizeof of a class member function pointer

Question

Let's say we have a class A

class A;

and these typedefs

typedef void (A::*a_func_ptr)(void);
typedef void (*func_ptr)(void);

My question is why sizeof(a_func_ptr) returns 16, while sizeof(func_ptr) returns 4 (as for any pointer on x86 system)?

For instance

int main(int argc, char *argv[])
{
  int a = sizeof(a_func_ptr);
  int b = sizeof(func_ptr);
}

Answer 1

My question is why sizeof(a_func_ptr) returns 16, while sizeof(func_ptr) returns 4 (as for any pointer on x86 system)?

Because pointer-to-members are implemented differently. They're not pointers under the hood. Some compilers, such as MSVC, implement them as struct with more than one members in it.

Read this interesting article:

• Pointers to member functions are very strange animals

Note that in some compilers, they might have same size. The bottomline is: they're compiler-dependent.