Constant Expressions

A constant expression is an expression which does not contain certain things. It may not have references to variables, string constants, or function calls. No subexpressions may have a type of spot, structure, union, array, or pointer. It may have numeric constants, character constants, and any operators that act on them, and the sizeof operator may appear with any operand.

Additionally, for constant expressions in preprocessor lines, you can test if a macro m has been defined by writing defined(m). This expression evaluates to 1 if a macro by that name has been defined, 0 if not.

The term "the constant zero" means a constant expression whose value is zero, not necessarily a numeric constant.

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