是否可以别名枚举类枚举器？ [英] Is it possible to alias an enum-class enumerator?

问题描述

给定一个C ++ 11枚举类，嵌套在几个长且名为丑陋的命名空间中：

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

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

可以使用枚举值的别名吗？使用clang ++ 3.5，可以做如下：

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，但是，抱怨。我无法复制其错误消息，因为我无法访问该代码，但它明确地抱怨使用using指令或声明。我也尝试过：

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;

但它也失败了。很抱歉，没有粘贴错误。

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

• 是否可以在标准C ++ 11中向枚举值声明别名，或者是否使用clang扩展名？

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

如果是，则正确的语法是什么？

If it is, what is the correct syntax?

推荐答案

中的枚举器

问题是，标准说你不应该引用一个枚举器使用指定 using-declaration 时，

7.3.3p7 / code>声明 [namespace.udecl] （n3337 ）

A using-declaration 不能为作用域的枚举器命名。

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

^{： clang 不接受上述两行; 这是一个相关的错误报告。}

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

参考枚举类本身的实际名称是完全正确的，但试图引用其中一个枚举器是错误的。

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.

因为枚举器不是类型，所以别名声明只能用于引用 type-name 上下文是不成形的。

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

---

使用 - 和别名声明的替代方法

一个有你想要的alias值初始化的任何名字的常数：

---

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`
}

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