Ada Programming/Attributes/'Val
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[edit] Description
X'Val(Y) is an Ada attribute where X is any discrete type and Y is an integer. This attribute represents the value of type X that has the same position number as Y. If no value of type X has a position number of Y, a CONSTRAINT_ERROR exception is raised.
Note that representation clauses do not effect position numbering. Whatever underlying value the enumerated value has, the position number will remain the same.
[edit] Example
-- Declare a discrete type and assume we know what -- position numbers the compiler will assign the values type My_Enum is (Enum1, Enum2, Enum3); -- Declare a discrete type and explicitly set the position numbers type My_Other_Enum is (Value1, Value2, Value3); for My_Other_Enum use (Value1 => 2, Value2 => 4, Value3 => 6 ); pragma Assert (My_Enum'Val(0) = Enum1); -- OK pragma Assert (My_Enum'Val(1) = Enum2); -- OK pragma Assert (My_Enum'Val(2) = Enum3); -- OK pragma Assert (My_Other_Enum'Val(0) = Value1); -- OK pragma Assert (My_Other_Enum'Val(1) = Value2); -- OK pragma Assert (My_Other_Enum'Val(2) = Value3); -- OK pragma Assert (My_Enum'Val(3) = Enum3); -- Wrong pragma Assert (My_Other_Enum'Val(2) = Value1); -- Wrong pragma Assert (My_Other_Enum'Val(4) = Value2); -- Wrong pragma Assert (My_Other_Enum'Val(6) = Value3); -- Wrong
