difference between typeids of template and non-template member function pointers

Question

I'm trying to figure out the root cause of a type conversion issue with pointers to member functions in C++ (VS2012).

I've narrowed it down to the difference in the typeid of a pointer to a template member function and a non-template member function. Consider the code:

#include <typeinfo>
#include <iostream>

class foo
{
    public:    
        template<int k>
        void func1() {}

        void func2() {}
};

typedef void (foo::*foofunc)();

int main()
{
    //this gives "int"
    std::cout<<typeid(&foo::func1<3>).name()<<std::endl;

    //this gives "void (__thiscall *)(void)"
    std::cout<<typeid(&foo::func2).name()<<std::endl;

    return 0;
}

func2 has the expected value. Why does a pointer to func1 have type int when both are member functions of the same class?

NOTE: This happens only on Windows.

@PaoloM: Yeah. I'm not sure why the name is obscure but at least they both seem to have the same type with GCC. — Cygnus, Nov 19 '15 at 15:15
What do you mean by obscure? By the way, this is from [cppreference](http://en.cppreference.com/w/cpp/language/typeid): There is no guarantee that the same std::type_info instance will be referred to by all evaluations of the typeid expression on the same type, although std::type_info::hash_code of those type_info objects would be identical, as would be their std::type_index. — Paolo M, Nov 19 '15 at 15:22

score 1 · Accepted Answer · edited May 23 '17 at 11:51

This clearly is a compiler bug in VS 2012. It looks closely related to #775434 "decltype is buggy in VS 2012" which links to this SO question. If this is indeed the bug, then you besides upgrading to VS 2015, you could try a workaround similar to this (src):

template <typename T> T msvc_decltype_helper(T); // Never implemented.
#define FN_DECLTYPE(...) decltype(::msvc_decltype_helper(__VA_ARGS__))

I.e. try what typeid(msvc_decltype_helper(&foo::func1<3>)).name() outputs.

difference between typeids of template and non-template member function pointers

1 Answers1