decltype and member function (not pointer) type

Question

struct C
{
    int Foo(int i) { return i; }

    typedef decltype(C::Foo) type;
};

Since there is no such type as a member function type (there isn't, is there?), I expect C::type to be int (int).

But the following won't compile using the Visual C++ 2012 RC:

std::function<C::type> f;

So what type is decltype(C::Foo)?

Answer 1

The code is ill-formed: there are only a few ways that a member function name (e.g. C::Foo) can be used, and this is not one of them (the complete list of valid uses can be found in the C++ language standard, see C++11 §5.1.1/12).

In the context of your example, the only thing you can really do is take the address of the member function, &C::Foo, to form a pointer to the member function, of type int (C::*)(int).

Since the code is ill-formed, the compiler should reject it. Further, it yields inconsistent results depending on how C::Foo is used; we'll look at the inconsistency below.

Please report a bug on Microsoft Connect. Alternatively, let me know and I am happy to report the issue.

---

If you have a type but you don't know what the type is, you can find out the name of the type by using it in a way that causes the compiler to emit an error. For example, declare a class template and never define it:

template <typename T>
struct tell_me_the_type;

Then later, you can instantiate this template with the type in which you are interested:

tell_me_the_type<decltype(C::Foo)> x;

Since tell_me_the_type hasn't been defined, the definition of x is invalid. The compiler should include the type T in the error it emits. Visual C++ 2012 RC reports:

error C2079: 'x' uses undefined struct 'tell_me_the_type_name<T>'
with
[
    T=int (int)
]

The compiler thinks that C::Foo is of type int (int). If that is the case, then the compiler should accept the following code:

template <typename T>
struct is_the_type_right;

template <>
struct is_the_type_right<int(int)> { };

is_the_type_right<decltype(C::Foo)> x;

The compiler does not accept this code. It reports the following error:

error C2079: 'x' uses undefined struct 'is_the_type_right<T>'
with
[
    T=int (int)
]

So, C::Foo both is of type int (int) and is not of type int (int), which violates the principle of noncontradiction. :-)

Answer 2

So what type is decltype(C::Foo)?

It's no type, since using just C::Foo is ill-formed.