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 (14.15), except that the only user-defined entities that can be referenced are conditional compilation symbols.