Ada Programming/Attributes/'Valid Scalars
Description
[edit | edit source]The 'Valid_Scalars attribute is intended to make it easier to check the validity of scalar subcomponents of composite objects. The attribute is defined for any prefix P which denotes an object. Prefix P can be any type except for tagged private or Unchecked_Union types. The value of the attribute is of type Boolean.
P'Valid_Scalars yields True if and only if the evaluation of C'Valid yields True for every scalar subcomponent C of P, or if P has no scalar subcomponents. Attribute 'Valid_Scalars is equivalent to attribute 'Valid for scalar types.
It is not specified in what order the subcomponents are checked, nor whether any more are checked after any one of them is determined to be invalid. If the prefix P is of a class-wide type T'Class (where T is the associated specific type), or if the prefix P is of a specific tagged type T, then only the subcomponents of T are checked; in other words, components of extensions of T are not checked even if T'Class (P)'Tag /= T'Tag.
The compiler will issue a warning if it can be determined at compile time that the prefix of the attribute has no scalar subcomponents.
Note: Valid_Scalars can generate a lot of code, especially in the case of a large variant record. If the attribute is called in many places in the same program applied to objects of the same type, it can reduce program size to write a function with a single use of the attribute, and then call that function from multiple places.