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
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.
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 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.
If X is volatile, the evaluation of X'Valid is considered
a read of X.
NOTE 3 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).
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