C语言 C 中是否允许空宏定义？他们的行为如何？
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Are empty macro definitions allowed in C? How do they behave?
提问by Zeta
Suppose the "empty" macro definition
假设“空”宏定义
#define FOO
Is it valid Standard C? If so, what is FOOafter this definition?
它是有效的标准 C 吗？如果是这样，FOO这个定义之后是什么？
回答by Zeta
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.
它只是一个可以扩展为什么都没有的宏。但是，既然已经定义了宏，您可以使用#if defined(或#ifdef)检查它是否已定义。
#define FOO
int main(){
FOO FOO FOO
printf("Hello world");
}
will expand to
将扩展到
int main(){
printf("Hello world");
}
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:
在某些情况下，这非常方便，例如您不想在发布版本中显示的附加调试信息：
/* 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");
/* ... */
}
Since the macro CONFIG_USE_DEBUG_MESSAGEShas 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-whileloop and you won't see the message.
由于宏CONFIG_USE_DEBUG_MESSAGES已定义，DEBUG_MSG(x)将扩展为print(x)，您将获得Entering main. 如果您删除#define,DEBUG_MSG(x)扩展为空do-while循环，您将看不到该消息。
回答by md5
Yes, empty define is allowed by the standard.
是的，标准允许空定义。
C11 (n1570), § 6.10 Preprocessing directives
control-line: # define identifier replacement-list new-line # define identifier lparen identifier-list(opt) ) replacement-list new-line # define identifier lparen ... ) replacement-list new-line # define identifier lparen identifier-list , ... ) replacement-list new-line replacement-list: pp-tokens(opt)
C11 (n1570), § 6.10 预处理指令
control-line: # define identifier replacement-list new-line # define identifier lparen identifier-list(opt) ) replacement-list new-line # define identifier lparen ... ) replacement-list new-line # define identifier lparen identifier-list , ... ) replacement-list new-line replacement-list: pp-tokens(opt)
A common utilisation is inclusion guards.
一个常见的用途是包含守卫。
#ifndef F_H
# define F_H
#endif
回答by smwikipedia
Empty macro definitions can also be used for self-documentation. The INin the code snippet below is a sample. The code and the comment are both quoted from the EDK II project.
空宏定义也可用于自文档。下面IN的代码片段是一个示例。代码和注释均引用自EDK II 项目。
//
// Modifiers for Data Types used to self document code.
// This concept is borrowed for UEFI specification.
//
///
/// Datum is passed to the function.
///
#define IN
typedef
EFI_STATUS
(EFIAPI *EFI_BLOCK_RESET)(
IN EFI_BLOCK_IO_PROTOCOL *This,
IN BOOLEAN ExtendedVerification
);
