启用和禁用代码功能的 C 宏

2022-01-11 00:00:00 c macros c++ c-preprocessor preprocessor-directive

我之前使用过一个代码库，它有一个用于启用和禁用代码段的宏系统.它看起来像下面这样:

I've used a code base before that had a macro system for enabling and disabling sections of code. It looked something like the following:

#define IN_USE      X
#define NOT_IN_USE  _

#if defined( WIN32 )
    #define FEATURE_A       IN_USE
    #define FEATURE_B       IN_USE
    #define FEATURE_C       NOT_IN_USE
#elif defined( OSX )
    #define FEATURE_A       NOT_IN_USE
    #define FEATURE_B       NOT_IN_USE
    #define FEATURE_C       IN_USE
#else
    #define FEATURE_A       NOT_IN_USE
    #define FEATURE_B       NOT_IN_USE
    #define FEATURE_C       NOT_IN_USE
#endif

然后功能的代码如下所示:

Then the code for the features would look like:

void DoFeatures()
{
#if USING( FEATURE_A )
    // Feature A code...
#endif

#if USING( FEATURE_B )
    // Feature B code...
#endif

#if USING( FEATURE_C )
    // Feature C code...
#endif

#if USING( FEATURE_D ) // Compile error since FEATURE_D was never defined
    // Feature D code...
#endif
}

我的问题(我不记得的部分)是如何定义USING"宏，以便在未将功能定义为IN_USE"或NOT_IN_USE"时出错?如果您忘记包含正确的头文件，可能会出现这种情况.

My question (the part I don't remember) is how to define the 'USING' macro so that it errors if the feature hasn't been defined as 'IN_USE' or 'NOT_IN_USE'? Which could be the case if you forget to include the correct header file.

#define USING( feature ) ((feature == IN_USE) ? 1 : ((feature == NOT_IN_USE) ? 0 : COMPILE_ERROR?))

推荐答案

你的例子已经实现了你想要的，因为 #if USING(x) 如果 USING 未定义.你在头文件中需要的只是

Your example already achieves what you want, since #if USING(x) will produce an error message if USING isn't defined. All you need in your header file is something like

#define IN_USE 1
#define NOT_IN_USE 0
#define USING(feature) feature

如果你想确保你也因为做类似的事情而得到一个错误

If you want to be sure that you also get an error just for doing something like

#if FEATURE

或

#if USING(UNDEFINED_MISPELED_FEETURE)

那么你可以这样做，比如说，

then you could do, say,

#define IN_USE == 1
#define NOT_IN_USE == 0
#define USING(feature) 1 feature

但您将无法防止这样的误用

but you won't be able to prevent such misuse as

#ifdef FEATURE

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