C++ Constant Expressions
A constant value is one that doesn't change. C++ provides two keywords to enable you to express the intent that an object is not intended to be modified, and to enforce that intent.
C++ requires constant expressions — expressions that evaluate to a constant — for declarations of:
Selectors in case statements
Bit-field length specification
The only operands that are legal in constant expressions are:
Values declared as const that are initialized with constant expressions
Nonintegral constants must be converted (either explicitly or implicitly) to integral types to be legal in a constant expression. Therefore, the following code is legal:
const double Size = 11.0; char chArray[(int)Size];
Explicit conversions to integral types are legal in constant expressions; all other types and derived types are illegal except when used as operands to the sizeof operator.
The comma operator and assignment operators cannot be used in constant expressions.