Can std::hash be used to hash function pointers?

Question

Can the C++11 std::hash type be used to hash function pointers? There is a hash partial specialization defined as

template <typename T> struct hash<T*>;

but since function pointers are different from other pointer types in C++ (e.g. they can't be cast to void*), I'm not sure whether it is safe to use it for types like int(*)() or void(*)(int, int).

Is this permitted? Is there any specific wording in the new ISO spec that supports or refutes this?

Thanks!

Answer 1

Great question. I don't know the answer for sure, and I'm happy to defer to anyone with better knowledge than me, but my thinking is that even though function pointers aren't the same as data pointers, they are pointers nonetheless: so the std::hash<T*> partial specialisation should be applied.

For what it's worth, the following compiles without warnings even with -pendantic in g++ 4.8.1 and clang 3.3, and works as expected:

#include <functional>
#include <iostream>

void func1(int) {}
void func2(int) {}

int main()
{
    typedef void (*func_type) (int);

    std::hash<func_type> hash;

    std::cout << hash(func1) << std::endl;
    std::cout << hash(func2) << std::endl;

}

I'd be really interested if anyone has any references to the standard to back this up though.