错误:宏名称必须是使用 #ifdef 0 的标识符

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

我有一个用 C++ 编写的应用程序的源代码，我只想用以下方式评论一些东西:

I have the source code of an application written in C++ and I just want to comment something using:

#ifdef 0
...
#endif

我得到了这个错误

错误:宏名必须是标识符

error: macro names must be identifiers

为什么会这样?

推荐答案

#ifdef 指令用于检查是否定义了预处理器符号. 标准(C11 6.4.2 Identifiers) 规定标识符不得以数字开头:

The #ifdef directive is used to check if a preprocessor symbol is defined. The standard (C11 6.4.2 Identifiers) mandates that identifiers must not start with a digit:

identifier:
    identifier-nondigit
    identifier identifier-nondigit
    identifier digit
identifier-nondigit:
    nondigit
    universal-character-name
    other implementation-defined characters>
nondigit: one of
    _ a b c d e f g h i j k l m
    n o p q r s t u v w x y z
    A B C D E F G H I J K L M
    N O P Q R S T U V W X Y Z
digit: one of
    0 1 2 3 4 5 6 7 8 9

使用预处理器阻塞代码的正确形式是:

The correct form for using the pre-processor to block out code is:

#if 0
: : :
#endif

你也可以使用:

#ifdef NO_CHANCE_THAT_THIS_SYMBOL_WILL_EVER_EXIST
: : :
#endif

但您需要确信这些符号不会被您自己的代码以外的代码无意中设置.换句话说，不要使用其他人也可能使用的 NOTUSED 或 DONOTCOMPILE 之类的东西.为了安全起见，应该首选 #if 选项.

but you need to be confident that the symbols will not be inadvertently set by code other than your own. In other words, don't use something like NOTUSED or DONOTCOMPILE which others may also use. To be safe, the #if option should be preferred.

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