Is it possible to alias an enum-class enumerator?

问题:

Given a C++11 enum class, nested inside several long- and ugly-named namespaces:

namespace
    long_and_ugly
{
    enum class
        colour
    {
        red,
        green,
        blue
    };
}

Can aliases be made of the enumeration values? With clang++ 3.5, it is possible to do what follows:

using long_and_ugly::colour; // take all the values into the current namespace
using long_and_ugly::colour::red; // take only 'red' into the current namespace

function_taking_colour_argument( red ); // instead of fully referring to the value

g++ 4.9, however, complains. I can't copy its error message because I can't access the code, but it explicitly complained about the usage of the using directive or declaration. I have also tried this:

using red = long_and_ugly::colour::red;

But it also failed. I'm sorry for not pasting the errors. Nevertheless, I believe you should be able to reproduce it.

---

Question(s)

• Is it possible to declare aliases to enumeration values in standard C++11, or was I using a clang extension?

• If it is, what is the correct syntax?

回答1:

Enumerators in using-declarations

The problem is that the standard says that you shall not refer to an enumerator inside an enum class when using specifying a using-declaration.

7.3.3p7 The using declaration [namespace.udecl] (n3337)

A using-declaration shall not name a scoped enumerator.

namespace N {
  enum class E { A };
}

using N::E;    // legal
using N::E::A; // ill-formed, violation of [namespace.udecl]p7

^{Note: clang does accept both lines above; here's a relevant bug report.}

It's perfectly fine to refer to the actual name of the enum class itself, but trying to refer to one of its enumerators is ill-formed.

---

Enumerators in alias-declarations

The standard says that an alias-declaration can only be used to refer to a type-name, since an enumerator isn't a type, using one in such context is ill-formed.

namespace N {
  enum class E { A };
}

using x = N::E;     // legal, `N::E` is a type
using y = N::E::A;  // ill-formed, `N::E::A` isn't a type

---

Alternatives to using- and alias-declarations

You could declare a constant having whatever-name-of-your-choice initialized with the value you'd like to "alias":

namespace N {
  enum class E { A };
}

constexpr N::E x = N::E::A;

int main () {
  N::E value = x; // semantically equivalent of `value = N::E::A`
}

回答2:

Sort of:

namespace long_and_ugly {
    enum class colour
    {
        red,
        green,
        blue
    };
}
const colour red = long_and_ugly::colour::red;