Ada 2005 Reference Manual
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.
NOTES
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 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).
