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?
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
DONOTCOMPILE which others may also use. To be safe, the
#if option should be preferred.
Use the following to evaluate an expression (constant 0 evaluates to false).
#if 0 ... #endif