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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 and has a valid representation. The value of this attribute is of the predefined type Boolean. 
NOTES
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19  Invalid data can be created in the following cases (not counting erroneous or unpredictable execution): 
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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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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).

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