C中是否允许使用空宏定义？他们的行为如何？ [英] Are empty macro definitions allowed in C? How do they behave?

问题描述

假设空宏定义

#define FOO

标准C是否有效？如果是这样，在此定义之后 FOO 是什么？

Is it valid Standard C? If so, what is FOO after this definition?

推荐答案

是只是一个扩展到什么都没有的宏。但是，既然已经定义了宏，则可以使用 #if defined （或 #ifdef ）检查宏是否具有

It is simply a macro that expands to, well, nothing. However, now that the macro has been defined you can check with #if defined (or #ifdef) whether it has been defined.

#define FOO

int main(){
    FOO FOO FOO
    printf("Hello world");
}

将扩展为

int main(){

    printf("Hello world");
}

在某些情况下，这非常方便，例如其他调试信息，您不想在发行版本中显示：

There are certain situations where this comes in very handy, for example additional debug information, which you don't want to show in your release version:

/* Defined only during debug compilations: */
#define CONFIG_USE_DEBUG_MESSAGES

#ifdef CONFIG_USE_DEBUG_MESSAGES
#define DEBUG_MSG(x) print(x)
#else
#define DEBUG_MSG(x) do {} while(0)
#endif

int main(){
    DEBUG_MSG("Entering main");
    /* ... */
}

由于宏 CONFIG_USE_DEBUG_MESSAGES 已定义， DEBUG_MSG（x）将扩展为 print（x），您将得到输入main 。如果删除 #define ，则 DEBUG_MSG（x）会扩展为空的 do -而循环时，您不会看到该消息。

Since the macro CONFIG_USE_DEBUG_MESSAGES has been defined, DEBUG_MSG(x) will expand to print(x) and you will get Entering main. If you remove the #define, DEBUG_MSG(x) expands to an empty do-while loop and you won't see the message.

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