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.)