C++ Constant Expressions

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.