什么用例需要#define没有令牌字符串？ [英] What use cases necessitate #define without a token-string?

问题描述

我在学习C之前遇到了 #define 预处理器指令，然后在一些读取的代码中遇到它。但除了使用它来定义常量的替换和定义宏，我没有真正低估的特殊情况下，它使用没有一个身体或令牌字符串。

I have encountered the #define pre-processor directive before while learning C, and then also encountered it in some code I read. But apart from using it to definite substitutions for constants and to define macros, I've not really understook the special case where it is used without a "body" or token-string.

以这一行为例：

#define OCSTR(X)

就这样！什么是使用这个或更好，什么时候使用 #define 必要？

Just like that! What could be the use of this or better, when is this use of #define necessary?

推荐答案

这在两种情况下使用。第一个和最常见的涉及
条件编译：

This is used in two cases. The first and most frequent involves conditional compilation:

#ifndef XYZ
#define XYZ
//  ...
#endif

但是它也可以是
用于像系统依赖的东西：

You've surely used this yourself for include guards, but it can also be used for things like system dependencies:

#ifdef WIN32
//  Windows specific code here...
#endif

更可能在命令行上定义，但
也可以在config.hpp文件中定义。）这通常
只涉及对象类宏（没有参数列表或
括号）。

(In this case, WIN32 is more likely defined on the command line, but it could also be defined in a "config.hpp" file.) This would normally only involve object-like macros (without an argument list or parentheses).

第二个是条件编译的结果。某事
like：

The second would be a result of conditional compilation. Something like:

#ifdef DEBUG
#define TEST(X) text(X)
#else
#define TEST(X)
#endif

允许写如下内容：

TEST(X);

将调用 DEBUG 如果
不是，则不执行任何操作。

which will call the function if DEBUG is defined, and do nothing if it isn't.

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