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:
X'Valid
{
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Yields True if and only if the object denoted by X is normal, has a valid
representation, and then, if the preceding conditions hold, the value
of X also satisfies the predicates
of the nominal
subtype of X. 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.
NOTE 1 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,
input,
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.
NOTE 2 {
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Determining whether X is normal and has a valid representation as part
of the evaluation of X'Valid is not considered to include an evaluation
of X; hence, it is not an error to check the validity of an object that
is invalid or abnormal. Determining whether X satisfies the predicates
of its nominal subtype can include an evaluation of X, but only after
it has been determined that X has a valid representation.
{
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If X is volatile, the evaluation of X'Valid is considered a read of X.
Reason: Since an implementation is not
allowed to add, remove, or reorder accesses to volatile objects, we have
to define X'Valid as a read so that it is implementable for most subtypes
as the value of the object is required.
NOTE 3 {
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The Valid attribute can 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 is still useful 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.
Extensions to Ada 83
X'Valid is new in Ada 95.
Wording Changes from Ada 95
{
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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
{
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The validity check now also includes a check of the predicate aspects
(see
3.2.4), if any, of the subtype of the
object.
Wording Changes from Ada 2012
{
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Corrigendum: Updated wording of the attributes X'Valid to use
the new term "satisfies the predicates" (see
3.2.4).
Also updated the notes to make sense when evaluating predicates and testing
validity of volatile objects.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
