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A constant expression gets evaluated at compile time, not run time, and can be used in any place that a constant can be used. The constant expression must evaluate to a constant that is in the range of representable values for that type. The operands of a constant expression can be integer constants, character constants, floating-point constants, enumeration constants, type casts, sizeof expressions, and other constant expressions.
Syntax
constant-expression:
conditional-expression
conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
expression:
assignment-expression
expression , assignment-expression
assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
The nonterminals for struct declarator, enumerator, direct declarator, direct-abstract declarator, and labeled statement contain the constant-expression nonterminal.
An integral constant expression must be used to specify the size of a bit-field member of a structure, the value of an enumeration constant, the size of an array, or the value of a case constant.
Constant expressions used in preprocessor directives are subject to several restrictions. They're known as restricted constant expressions. A restricted constant expression can't contain sizeof expressions, enumeration constants, type casts to any type, or floating-type constants. It can, however, contain the special constant expression defined ( identifier ).