使用 #define 重新定义 enum 枚举器
我在 C 头文件中发现了一些我不知道其用途的东西。例如，在文件 bits/socket.h 中，有一个枚举类型 enum __socket_type，但在每个枚举器后面都有一个定义相同的宏。示例：
enum __socket_type
{
SOCK_STREAM = 1,
#define SOCK_STREAM SOCK_STREAM
...
};
我一直无法找出它的用途。请赐教。我什至不知道如何形成正确的问题来查询谷歌或该网站的搜索框。
I have spotted something in C header files what I can't figure out what is for. For example in file bits/socket.h there is an enumeration type enum __socket_type, but after every enumerator there is a define macro which defines the same. Example:
enum __socket_type
{
SOCK_STREAM = 1,
#define SOCK_STREAM SOCK_STREAM
...
};
I have been unable to find out what this is for. Please enlighten me. I don't even know how to form right question for querying google nor this site search box.
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预处理器宏永远不会递归扩展，因此这样的#define 所做的就是将名称保留在使用的地方。当您想要进行预处理器功能测试时，这些东西非常有用。
可用于随后有条件地编译一些代码。
编辑：因此，这将更简洁的枚举方法（没有冲突和范围界定的隐式值）与预处理器测试结合起来。
A prepreprocessor macro will never expand recursively, so what such a
#definedoes is leave the name in place whereever it is used. Such things are useful when you want to have a preprocessor feature test.can be used to conditionally compile some code afterwards.
Edit: So this combines the cleaner approach of enumerations (implicit values without collisions and scoping) with preprocessor tests.
我唯一能想到的是，因为人们看到一个全大写的常量，比如
NUM_FILES，他们会认为这是一个宏，并试图这样写：现在通常这会失败，但是如果你编写
#define NUM_FILES NUM_FILES，它的行为就像预处理器和IDE的宏以及代码本身的枚举。The only thing I can think of is because people see a constant in all-caps, say
NUM_FILES, they'll think it's a macro and are tempted to write this:Now normally this would fail, but if you write
#define NUM_FILES NUM_FILESit behaves as a macro for the preprocessor and IDE's and as an enum for the code itself.我怀疑 IDE 或其他工具可以理解符号是以某种方式定义的。
I would suspect it's for IDEs or other tools to understand that a symbol is defined in some way.