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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.

    NOTES

  4. (19) Invalid data can be created in the following cases (not counting erroneous or unpredictable execution):
    1. an uninitialized scalar object,
    2. the result of an unchecked conversion,
    3. input,
    4. interface to another language (including machine code),
    5. aborting an assignment,
    6. disrupting an assignment due to the failure of a language-defined check, See section 11.6 Exceptions and Optimization, and
    7. use of an object whose Address has been specified.

  1. (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.

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