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A constant expression must evaluate to an arithmetic constant
expression, a null pointer constant, an address constant, or an
address constant plus or minus an integral constant expression.
An integral constant expression must involve only integer
constants, enumeration constants, character constants, sizeof
expressions, and casts to integral types. You cannot include the
array subscripting (),
member access (.
and ->), and
address of ()
operators in integral constant expressions. An integral constant
expression with the value 0, or such an expression cast to type
void *, is called
a null pointer constant. An address
constant is a pointer to an object of static storage
duration or to a function designator.
Further, you cannot use a function call, an increment or a
decrement operator, or indirection or assignment operations in a
constant expression.
Constant expressions are usually used for "allocation" type
operations. An example of this is array allocation. The size of
an array is given as a constant expression.
