What am I doing wrong with enable_if and has_member?

Question

I think I've been staring at this for too long or something, but I can't find my error here:

struct
{
    bool empty() const
    {
       return true;
    }
} hasEmpty;

template<typename T>
struct has_empty
{
private:
    template<typename U, U>
    class check {};

    template<typename C>
    static char f(check<void (C::*)() const, &C::empty> *);

    template<typename C>
    static long f(...);

public:
    static const bool value = (sizeof(f<T>(nullptr)) == sizeof(char));
};

template<typename T>
typename std::enable_if<has_empty<T>::value>::type foo(const T& t)
{

}

void x()
{
    foo(hasEmpty);
}

Visual Studio 2012 reports:

error C2893: Failed to specialize function template 'std::enable_if<has_empty<T>::value>::type foo(const T &)'
1>          With the following template arguments:
1>          '<unnamed-type-hasEmpty>'

(Note, I really like the new C++11 version of this test as described here, but VS2012 doesn't support constexpr, yet.)

Answer 1

Your hasEmpty::empty method returns bool:

struct 
{
    bool empty() const
    {
       return true;
    }
} hasEmpty;

But your trait uses a member function pointer that returns void, that substitution will always fail. You should change this:

template<typename C>
ctatic char f(check<void (C::*)() const, &C::empty> *);

For this:

template<typename C>
static char f(check<bool (C::*)() const, &C::empty> *);

That compiles for me.