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13.9.2 The Valid Attribute

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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

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For a prefix X that denotes a scalar object (after any implicit dereference), the following attribute is defined: 
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X'Valid
Yields True if and only if the object denoted by X is normal, has a valid representation, and the predicate of the nominal subtype of X evaluates to True. The value of this attribute is of the predefined type Boolean. 
NOTES
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22  Invalid data can be created in the following cases (not counting erroneous or unpredictable execution): 
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an uninitialized scalar object,
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the result of an unchecked conversion,
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input,
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interface to another language (including machine code),
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aborting an assignment,
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disrupting an assignment due to the failure of a language-defined check (see 11.6), and
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use of an object whose Address has been specified. 
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23  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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24  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).

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