Scope resolution operator on enums a compiler-specific extension?

问题

On this question, there's an answer that states:

You can use typedef to make Colour enumeration type accessible without specifying it's "full name".
typedef Sample::Colour Colour;
Colour c = Colour::BLUE;

That sounds correct to me, but someone down-voted it and left this comment:

Using the scope resolution operator :: on enums (as in "Colour::BLUE") is a compiler-specific extension, not standard C++

Is that true? I believe I've used that on both MSVC and GCC, though I'm not certain of it.

回答1:

I tried the following code:

enum test
{
    t1, t2, t3
};

void main() 
{
    test t = test::t1;
}

Visual C++ 9 compiled it with the following warning:

warning C4482: nonstandard extension used: enum 'test' used in qualified name

Doesn't look like it's standard.

回答2:

That is not standard.

In C++11, you will be able to make scoped enums with an enum class declaration.

With pre-C++11 compilers, to scope an enum, you will need to define the enum inside a struct or namespace.

回答3:

In standard c++, things to the left of "::" must be a class or namespace, enums don't count.

回答4:

This is not allowed in C++98. However, staring from C++11 you can optionally use scope resolution operator with "old-style" enums

enum E { A };

int main()
{
  A;    // OK
  E::A; // Also OK
}

Both ways of referring to A are correct in C++11 and later.

回答5:

What you can do to get around it is to create a namespace that's the same name as the enumeration. That will effectively add the enumeration values into their own scope and you can use the name of the enumeration/namespace to refer to them. Of course it only works for enumerations that would otherwise exist in the global (or another namespace) scope.

There's also an article on this issue somewhere.

来源：https://stackoverflow.com/questions/441552/scope-resolution-operator-on-enums-a-compiler-specific-extension

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