什么是 Objective-C 中的 typedef 枚举? [英] What is a typedef enum in Objective-C?

问题描述

我不认为我从根本上理解 enum 是什么，以及何时使用它.

I don't think I fundamentally understand what an enum is, and when to use it.

例如:

typedef enum {
    kCircle,
    kRectangle,
    kOblateSpheroid
} ShapeType;

这里真正声明的是什么?

What is really being declared here?

推荐答案

这里声明了三件事:声明了一个匿名枚举类型，ShapeType 被声明为该匿名枚举的 typedef，以及三个名称 kCircle、kRectangle 和 kOblateSpheroid 被声明为整数常量.

Three things are being declared here: an anonymous enumerated type is declared, ShapeType is being declared a typedef for that anonymous enumeration, and the three names kCircle, kRectangle, and kOblateSpheroid are being declared as integral constants.

让我们分解一下.在最简单的情况下，枚举可以声明为

Let's break that down. In the simplest case, an enumeration can be declared as

enum tagname { ... };

这声明了一个带有标签 tagname 的枚举.在C 和Objective-C(但不是 C++)中，对这个的任何引用都必须以enum关键字开头.例如:

This declares an enumeration with the tag tagname. In C and Objective-C (but not C++), any references to this must be preceded with the enum keyword. For example:

enum tagname x;  // declare x of type 'enum tagname'
tagname x;  // ERROR in C/Objective-C, OK in C++

为了避免到处使用enum关键字，可以创建一个typedef:

In order to avoid having to use the enum keyword everywhere, a typedef can be created:

enum tagname { ... };
typedef enum tagname tagname;  // declare 'tagname' as a typedef for 'enum tagname'

这可以简化为一行:

typedef enum tagname { ... } tagname;  // declare both 'enum tagname' and 'tagname'

最后，如果我们不需要能够将 enum tagname 与 enum 关键字一起使用，我们可以制作 enum匿名并且只用 typedef 名称声明它:

And finally, if we don't need to be able to use enum tagname with the enum keyword, we can make the enum anonymous and only declare it with the typedef name:

typedef enum { ... } tagname;

现在，在本例中，我们将 ShapeType 声明为匿名枚举的 typedef 名称.ShapeType 实际上只是一个整数类型，并且应该只用于声明包含声明中列出的值之一的变量(即 kCircle、kRectangle 和 kOblateSpheroid).但是，您可以通过强制转换为 ShapeType 变量分配另一个值，因此在读取枚举值时必须小心.

Now, in this case, we're declaring ShapeType to be a typedef'ed name of an anonymous enumeration. ShapeType is really just an integral type, and should only be used to declare variables which hold one of the values listed in the declaration (that is, one of kCircle, kRectangle, and kOblateSpheroid). You can assign a ShapeType variable another value by casting, though, so you have to be careful when reading enum values.

最后，kCircle、kRectangle 和 kOblateSpheroid 被声明为全局命名空间中的整数常量.由于没有指定特定的值，它们被分配给从 0 开始的连续整数，所以 kCircle 是 0，kRectangle 是 1，而 kOblateSpheroid 是2.

Finally, kCircle, kRectangle, and kOblateSpheroid are declared as integral constants in the global namespace. Since no specific values were specified, they get assigned to consecutive integers starting with 0, so kCircle is 0, kRectangle is 1, and kOblateSpheroid is 2.

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