switch
labels must be constant expressions, they have to be evaluated at compile time. If you want to branch on run-time values, you must use an if
.
A const
-qualified variable is not a constant expression, it is merely a value you are not allowed to modify.
The form of integer constant expressions is detailed in 6.6 (6) [C99 and the n1570 draft of the C2011 standard]:
6 An integer constant expression shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants,
sizeof
expressions whose results are integer constants,_Alignof
expressions, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to thesizeof
or_Alignof
operator.
The restriction that only sizeof
expressions whose result is an integer constant are allowed rules out sizeof
expressions whose operand is a variable length array.