Error: macro names must be identifiers using #ifde

问题:

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

#ifdef 0
...
#endif

And I get this error

error: macro names must be identifiers

Why is this happening?

回答1:

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

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

#if 0
: : :
#endif

You can also use:

#ifdef NO_CHANCE_THAT_THIS_SYMBOL_WILL_EVER_EXIST
: : :
#endif

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.

回答2:

Use the following to evaluate an expression (constant 0 evaluates to false).

#if 0
 ...
#endif

回答3:

This error can also occur if you are not following the marco rules

Like

#define 1K 1024 // Macro rules must be identifiers error occurs

Reason: Macro Should begin with a letter, not a number

Change to

#define ONE_KILOBYTE 1024 // This resolves 

回答4:

#ifdef 0
...
#endif

#ifdef expect a macro rather than expression when using constant or expression

#if 0
...
#endif

or

#if !defined(PP_CHECK) || defined(PP_CHECK_OTHER)
..
#endif

if #ifdef is used the it reports this error

#ifdef !defined(PP_CHECK) || defined(PP_CHECK_OTHER)
..
#endif

Where #ifdef expect a macro rather than macro expresssion

回答5:

Note that you can also hit this error if you accidentally type:

#define <stdio.h>

...instead of...

#include <stdio.>