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# C++ Constant Expressions
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](../cpp/types-of-expressions.md)