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C++ Constant Expressions
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.