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C++ requires constant expressions — expressions that evaluate to a constant — for declarations of:
Array bounds
Selectors in case statements
Bit-field length specification
Enumeration initializers
The only operands that are legal in constant expressions are:
Literals
Enumeration constants
Values declared as const that are initialized with constant expressions
sizeof 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.