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13.9.2 The Valid Attribute

1
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

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For a prefix X that denotes a scalar object [(after any implicit dereference)], the following attribute is defined: 
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X'Valid
Yields True if and only if the object denoted by X is normal and has a valid representation. The value of this attribute is of the predefined type Boolean. 
3.a
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.
NOTES
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20  Invalid data can be created in the following cases (not counting erroneous or unpredictable execution): 
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21  X'Valid is not considered to be a read of X; hence, it is not an error to check the validity of invalid data.
13/2
22  {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).
13.a
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.
13.b
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.
13.c/2
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

13.d
{extensions to Ada 83} X'Valid is new in Ada 95. 

Wording Changes from Ada 95

13.e/2
{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. 

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