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C++ Constant Expressions

  • Article

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:

  • 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.

See also

Types of Expressions