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
2
For a prefix
X that denotes a scalar object [(after any implicit dereference)], the
following attribute is defined:
3
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.
4
20 Invalid
data can be created in the following cases (not counting erroneous or
unpredictable execution):
5
- an uninitialized scalar object,
6
- the result of an unchecked conversion,
7
8
- interface to another language
(including machine code),
9
10
- disrupting an assignment due
to the failure of a language-defined check (see 11.6),
and
11
- use of an object whose Address
has been specified.
12
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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