什么是枚举类，为什么要关心？ [英] What's an enum class and why should I care?

问题描述

对于一个从来没有写过C ++ 11行的人来说，目前还没有机会在C ++ 11中编程，你可以在一个简短的段落中告诉我：

什么是枚举类，为什么需要它？

解决方案

枚举类被称为范围枚举。它可以防止枚举出现的命名空间与枚举器的名称。

在C ++ 03中，您可以通过将 enum 在一个专门的类中。也许这是语法的来源，这有点混乱。

另一个区别是这种类型的枚举器不会隐式转换为 int （ static_cast< int> 是必需的）。这可能很少需要，但它可以安全地重载一个使用 int 参数的函数，一个取$ 枚举类型。您可以确定不会意外调用 int 。或者您可以使用专用的运算符函数定义伪整数类型，并确保内置运算符不会干扰。

$ b $这有点令人讨厌，这两个不相关的差异在同一个包中，并且您无法获得没有隐式转换的枚举枚举，但通常这两个更改都是Good Things和枚举类是C ++ 11中的一个很好的默认做法。

编辑：范围枚举定义如下：

 枚举类duck {huey，dewey，louie}; 
  

，必须与范围解析运算符 :: 如下所示：

  duck culprit = duck :: huey; //或auto culprit以避免冗余
  

请注意， 运算符也可以使用C ++ 03无范围的枚举，所以上面的第二行即使第一个缺少 class 。

这可能是过多的细节，但 class 不会进入详细说明型的说明符，如果向前声明枚举类型，如

  void quack（enum duck who）; //不是enum class
  

然而，C ++ 11中有一个新构造， opaque-enum-declaration ，其中包含类关键字并定义完整的类型。

枚举鸭子$ pre> // duck被声明为不完整的类型
enum class duck; //鸭现在完成类型;底层类型默认为int


关键字 struct 可以代替 class ，没有语义差异。

For one who has never written a line of C++11, and who has, at the moment, no opportunity to program in C++11, can you, in one short paragraph., tell me:

What is an "enum class" and why do we need it?

解决方案

enum class is called a scoped enumeration. It prevents polluting the namespace where the enumeration appears with the names of the enumerators.

In C++03, you could do effectively the same thing by putting the enum inside a dedicated class. Perhaps that's the source of the syntax, which is a bit confusing.

Another difference is that the enumerators of such a type don't convert implicitly to int (static_cast<int> is required). This may be seldom needed but it makes it safe to overload a function taking an int argument with one taking enum type. You can be sure the int won't be called by accident. Or you can define pseudo-integral types with dedicated operator functions, and be sure that built-in operators won't interfere.

It's a bit annoying that these two unrelated differences come in the same package, and that you can't get an unscoped enumeration with no implicit conversion, but generally both changes are Good Things and enum class is a good default practice in C++11.

EDIT: A scoped enumeration is defined like this:

enum class duck { huey, dewey, louie };

and must be used with the scope resolution operator :: like this:

duck culprit = duck::huey; // or "auto culprit" to avoid redundancy

Note that the :: operator also works with C++03 unscoped enumerations, so the second line above would work even if the first was missing class.

This might be excessive detail, but class does not go into the elaborated-type-specifier if forward declaring the enumerated type, as in

void quack( enum duck whom ); // not "enum class"

However, there is a construct new in C++11, the opaque-enum-declaration, which does include the class keyword and defines a complete type.

enum duck; // duck is declared as incomplete type
enum class duck; // duck is now complete type; underlying type defaults to int

The keyword struct can be substituted for class with no semantic difference.

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