# C Constant Expressions

**Visual Studio .NET 2003**

A constant expression is 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 additional restrictions. Consequently, they are known as "restricted constant expressions." A restricted constant expression cannot contain **sizeof** expressions, enumeration constants, type casts to any type, or floating-type constants. It can, however, contain the special constant expression defined**(***identifier***)**.