在这种情况下，匿名枚举应该使用？ [英] in which situations anonymous enum should be used?

问题描述

在C和C ++ 枚举可以与标签来定义

In both c and c++ enum could be defined with tag

enum e_smth {
    smth_one,
    smth_two,
    smth_err
};

或无标签

enum {
    smth_one,
    smth_two,
    smth_err
};

---

如果有人用标签来定义它是有道理的开关在C和C ++语句：

e_smth some_var;
....//do some stuff with some_var
switch (some_var)
{
case smth_one:
break;
case smth_two:
break;
}

将产生 -Wswitch 警告这是否会用gcc或g ++编译。

will produce -Wswitch warning if this will be compiled with gcc or g++.

这是有道理的函数的声明和变量初始化在C ++：

It makes sense in functions declarations and variables initialization in c++ :

e_smth var;
var=99;

如果使用g ++编译会产生 -fpermissive 错误。

Will produce -fpermissive error if compiled with g++.

这两种类型的有或无标签可以被用作一个文件的#define 宏没有参数。

Both types with or without tag could be used as one file #define macro without parameters.

更新

可以作为一个文件的#define 宏不带参数

could be used as one file #define macro without parameters

意味着：如果不希望编写的#define MAX 1000 中的文件并添加MAX全球使用枚举{MAX = 1000} 刚刚在文件范围

meant : Instead of writing #define MAX 1000 in file and adding MAX to global use enum { MAX=1000 } just in file scope

和有关匿名枚举，我发现了什么只有一个用例：
定义，就像的typedef枚举{A，B，C} some_t; 使得它像标签枚举工作

And what about anonymous enums, I've found just one use cases: definition like typedef enum { a,b,c } some_t; makes it work like enum with tag

如果我还没有描述的所有合理使用的情况下，应该使用什么匿名枚举？

if I haven't yet described all reasonable use cases, what for anonymous enums should be used?

推荐答案

在C（而不是在C ++），枚举可以是[AB]用于定义 INT 常量。

In C (but not in C++), enum can be [ab]used to define int constants.

例如，鉴于这一声明：

const int MAX = 1024;

MAX 不是恒定的前pression，这是一个只读对象的名称。这意味着你不能在标签的情况下使用它，作为一个数组的大小在文件范围内或者与静态，或以任何其他情况下需要不断前$声明p $ pssion。

MAX is not a constant expression, it's the name of a read-only object. That means you can't use it in a case label, as the size of an array declared at file scope or with static, or in any other context requiring a constant expression.

但是，如果你写的：

enum { MAX = 1024 };

然后 MAX 的是的类型的常量前pression INT ，可用在任何情况下，你可以使用常数 1024 。

then MAX is a constant expression of type int, usable in any context where you could use the constant 1024.

当然，你也可以写：

#define MAX 1024

但也有缺点使用preprocessor：标识符的作用域，将被赋予一个普通的申报方式，例如：

but there are disadvantages to using the preprocessor: the identifier isn't scoped the way it would be given an ordinary declaration, for example.

的缺点是，这样一个恒定的只能是类型 INT 的

The drawback is that such a constant can only be of type int.

C ++有不同的规则;枚举常量是枚举类型，而不是 INT 的，但您可以使用声明的常量对象恒定前pressions（只要初始值是恒定的前pression）。

C++ has different rules; enumeration constants are of the enumerated type, not int, but you can use declared constant objects as constant expressions (as long as the initializer is a constant expression).

要解决原来的问题，当您使用枚举声明来创建这样的常量，有一个在具有或者标签或一个typedef没有意义的，因为你从不使用该类型本身。

To address the original question, when you use an enum declaration to create constants like this, there's no point in having either a tag or a typedef, since you'll never use the type itself.

背景：

enum foo { zero, one, two };
enum foo obj = two;

创建类型为枚举富和常量零，有一个和两个。在C中，常量总是键入 INT ，这是无可否认奇，和初始化 OBJ 涉及的隐转化率从 INT 到枚举富。

creates a type enum foo and constants zero, one, and two. In C, the constants are always of type int, which is admittedly odd, and the initialization of obj involves an implicit conversion from int to enum foo.

在C ++中，键入枚举富，也可以简称为富和常量的键入枚举富（这是兼容的部分的整数类型，不一定 INT ）。

In C++, the type enum foo can also be referred to as just foo, and the constants are of type enum foo (which is compatible with some integer type, not necessarily int).

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