Pre-processing expressionsPre-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: pp-or-expression: pp-and-expression: pp-equality-expression: pp-unary-expression: pp-primary-expression: 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 (§7.19), except that the only user-defined entities that can be referenced are conditional compilation symbols.
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