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13.9.2 The Valid Attribute

The Valid attribute can be used to check the validity of data produced by unchecked conversion, input, interface to foreign languages, and the like. 

Static Semantics

For a prefix X that denotes a scalar object [(after any implicit dereference)], the following attribute is defined: 
{AI05-0153-3} Yields True if and only if the object denoted by X is normal, and has a valid representation, and the predicate of the nominal subtype of X evaluates to True. The value of this attribute is of the predefined type Boolean.
Ramification: Having checked that X'Valid is True, it is safe to read the value of X without fear of erroneous execution caused by abnormality, or a bounded error caused by an invalid representation. Such a read will produce a value in the subtype of X.
23  Invalid data can be created in the following cases (not counting erroneous or unpredictable execution): 
an uninitialized scalar object,
the result of an unchecked conversion,
interface to another language (including machine code),
aborting an assignment,
disrupting an assignment due to the failure of a language-defined check (see 11.6), and
use of an object whose Address has been specified. 
24  X'Valid is not considered to be a read of X; hence, it is not an error to check the validity of invalid data.
25  {AI95-00426-01} The Valid attribute may be used to check the result of calling an instance of Unchecked_Conversion (or any other operation that can return invalid values). However, an exception handler should also be provided because implementations are permitted to raise Constraint_Error or Program_Error if they detect the use of an invalid representation (see 13.9.1).
Ramification: If X is of an enumeration type with a representation clause, then X'Valid checks that the value of X when viewed as an integer is one of the specified internal codes.
Reason: Valid is defined only for scalar objects because the implementation and description burden would be too high for other types. For example, given a typical run-time model, it is impossible to check the validity of an access value. The same applies to composite types implemented with internal pointers. One can check the validity of a composite object by checking the validity of each of its scalar subcomponents. The user should ensure that any composite types that need to be checked for validity are represented in a way that does not involve implementation-defined components, or gaps between components. Furthermore, such types should not contain access subcomponents.
This paragraph was deleted.{AI95-00114-01} Note that one can safely check the validity of a composite object with an abnormal value only if the constraints on the object and all of its subcomponents are static. Otherwise, evaluation of the prefix of the attribute_reference causes erroneous execution (see 4.1).

Extensions to Ada 83

X'Valid is new in Ada 95.

Wording Changes from Ada 95

{AI95-00426-01} Added a note explaining that handlers for Constraint_Error and Program_Error are needed in the general case of testing for validity. (An implementation could document cases where these are not necessary, but there is no language requirement.) 

Wording Changes from Ada 2005

{AI05-0153-3} The validity check now also includes a check of the predicate aspects (see 3.2.4), if any, of the subtype of the object. 

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