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 and has a valid representation. 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.
19 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.
20 X'Valid is not considered to be a read
of X; hence, it is not an error to check the validity of invalid data.
21 {
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.
Extensions to Ada 83
{
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.
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