Ada Programming/Attributes/'Valid

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Contents 1 Description 2 Example 3 See also 3.1 Wikibook 3.2 Ada Reference Manual 3.3 Ada Quality and Style Guide 3.4 Ada Rationale

[edit] Description

The Valid attribute can be used with an object of any scalar type (that is, numeric or enumeration types) to know whether its value is valid (e.g. not out-of-range, etc). The result is always True or False; neither CONSTRAINT_ERROR nor any other exception are ever raised.

[edit] Example

-- Declare a discrete type and explicitly set the position numbers
type My_Enum is (Value1, Value2, Value3);
for My_Enum use (Value1 => 2, Value2 => 4, Value3 => 6 );

Result           : Natural;
Enum_Var         : My_Enum;
Sneaky_Back_Door : Integer;
for Enum_Var'Address use Sneaky_Back_Door'Address;

...

if not Result'Valid then
   -- Result is out-of-range, it has a negative value
   Result := Natural'First;
end if;
...
-- Assign a bad integer value to the enumerated type variable.
Sneaky_Back_Door := 1;
...
if not Enum_Var'Valid then
   -- Enum_Var contains a bad value
   Enum_Var := My_Enum'First;
end if;

[edit] See also

[edit] Wikibook

• Ada Programming
• Ada Programming/Attributes

[edit] Ada Reference Manual

• 13.9.2 The Valid Attribute (Annotated)

[edit] Ada Quality and Style Guide

• 5.9.1 Unchecked Conversion — Consider using the 'Valid attribute to check the validity of scalar data.
• 5.9.7 Direct_IO and Sequential_IO — Use the 'Valid attribute to check the validity of scalar values obtained through Ada.Direct_IO and Ada.Sequential_IO.
• 6.3.3 The Abort Statement — In the presence of the abort statement, consider using the 'Valid attribute to check the validity of scalar data.

[edit] Ada Rationale

• h.1.1 The Valid Attribute