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C++ Constant Expressions

  • Article

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

Syntax

  • constant-expression :
    conditional-expression

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.