2.5.2 Pre-processing expressions

Pre-processing expressions can occur in #if and #elif directives. The operators !, ==, !=, && and || are permitted in pre-processing expressions, and parentheses may be used for grouping.

pp-expression:

whitespace_opt   pp-or-expression   whitespace_opt

pp-or-expression:

pp-and-expression
pp-or-expression   whitespace_opt   ||   whitespace_opt   pp-and-expression

pp-and-expression:

pp-equality-expression
pp-and-expression   whitespace_opt   &&   whitespace_opt   pp-equality-expression

pp-equality-expression:

pp-unary-expression
pp-equality-expression   whitespace_opt   ==   whitespace_opt   pp-unary-expression
pp-equality-expression   whitespace_opt   !=   whitespace_opt   pp-unary-expression

pp-unary-expression:

pp-primary-expression
!   whitespace_opt   pp-unary-expression

pp-primary-expression:

true
false
conditional-symbol
(   whitespace_opt   pp-expression   whitespace_opt   )

When referenced in a pre-processing expression, a defined conditional compilation symbol has the Boolean value true, and an undefined conditional compilation symbol has the Boolean value false.

Evaluation of a pre-processing expression always yields a Boolean value. The rules of evaluation for a pre-processing expression are the same as those for a constant expression (Section 7.15), except that the only user-defined entities that can be referenced are conditional compilation symbols.