包含重复值的C ++ 11枚举类 [英] C++11 Enum class containing duplicate values

问题描述

经典的C ++枚举没有简单的方法来检测重复值。

The classic C++ enums don't have a straight forward method to detect duplicate values.

新C ++ 11中的此问题是否已解决枚举类？

Is this issue addressed in the new C++11 enum class?

enum class ConnectionState : uint32_t
{
    Connecting,
    Reconnecting = 2,
    Disconnecting,
    LocalConnection,
    NoNetwork = 2,
    WifiNetwork,
    Last
}

推荐答案

当前无法检测或阻止一个枚举中的多个相同枚举值枚举。

There is currently no way to detect or prevent multiple identical enum values in an enum.

反射工作组正在研究如何向语言中添加反射（C ++代码自省C ++代码的能力）。在反射所涵盖的一长串内容中，有一个简短的列表正在处理，在那个短列表中，可以检查编译时枚举的值。

The reflection working group is working on how to add reflection -- the ability for C++ code to introspect C++ code -- to the language. In the long list of stuff that reflection covers, there is a short list being worked on, and in that short list, examining the values of an enumeration at compile time is there.

N4428 包含有关枚举反射。 那里有一些部分实现。

N4428 contains a proposal for enum reflection. There are some partial implementations out there.

根据N4428，检测重复项将很容易。您可以在编译时获得枚举值的数量及其值。只需按顺序创建所有枚举值的包，然后测试它们是否唯一。然后将该测试的结果放入 static_assert 。

Under N4428, detecting duplicates would be easy. You can get the number of enumeration values, and their value, all at compile time. Simply create a pack of all the enum values in order, and test that they are unique. Then toss the result of that test into a static_assert.

最终结果可能是：

template<class E>
constexpr bool all_values_unique(); // todo

static_assert( all_values_unique<ConnectionState>(), "Duplicate enum value detected" );

在将反射提案添加到C ++之前，这是不可能的。

Prior to something like that reflection proposal being added to C++, this is not possible.

您可以使用宏来伪造它-具有一个既可以创建枚举又可以创建反射特性信息的宏-然后编写 all_values_unique 使用反射特征信息。这样做的好处是，如果/当标准和/或编译器获得所需的反射功能时，可能很容易剥离宏。

You can fake it using macros -- have a macro that both creates your enum and creates reflection traits information about it -- then write all_values_unique that uses the reflection traits information. This has the advantage that if/when the standard and/or your compiler gets the reflection features needed, it may be easy to strip out your macros.

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