Integer constant expressions

An integer compile-time constant is a value that is determined during compilation and cannot be changed at run time. An integer compile-time constant expression is an expression that is composed of constants and evaluated to a constant.

An integer constant expression is an expression that is composed of only the following:
  • literals
  • enumerators
  • const variables
  • static data members of integral or enumeration types
  • casts to integral types
  • sizeof expressions, where the operand is not a variable length array

The sizeof operator applied to a variable length array type is evaluated at run time, and therefore is not a constant expression.

You must use an integer constant expression in the following situations:
  • In the subscript declarator as the description of an array bound.
  • After the keyword case in a switch statement.
  • In an enumerator, as the numeric value of an enumeration constant.
  • In a bit-field width specifier.
  • In the preprocessor #if statement. (Enumeration constants, address constants, and sizeof cannot be specified in a preprocessor #if statement.)