Error: macro names must be identifiers using #ifdef 0

Question

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?

Answer 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.