Undefined values: Difference between revisions

=={{header|Ada}}==

{{works with|GNAT}}

Ada language provides attribute 'Valid used to check if a scalar value is valid. An invalid value may appear as a result of unchecked type conversion, input, access through a dangling pointer etc.

The language also provides the configuration pragma Normalize_Scalars which instructs the compiler to initialize uninitialized scalars with values, which when possible, would have the attribute 'Valid false.

This pragma is required to be applied to the whole partition, which would require recompilation of the run-time library. For this reason, the presented example uses another pragma Initialize_Scalars.

This one has the effect similar to Normalize_Scalars, but is [[GNAT]]-specific:

<lang Ada>

pragma Initialize_Scalars;

with Ada.Text_IO; use Ada.Text_IO;

procedure Invalid_Value is

type Color is (Red, Green, Blue);

X : Float;

Y : Color;

begin

if not X'Valid then

Put_Line ("X is not valid");

end if;

X := 1.0;

if X'Valid then

Put_Line ("X is" & Float'Image (X));

end if;

if not Y'Valid then

Put_Line ("Y is not valid");

end if;

Y := Green;

if Y'Valid then

Put_Line ("Y is " & Color'Image (Y));

end if;

end Invalid_Value;

</lang>

Sample output:

<pre>

X is not valid

X is 1.00000E+00

Y is not valid

Y is GREEN

</pre>

Note that some types are always initialized valid. E.g. pointers, which are formally are not scalar, are initialized null. Another example are scalar types which representation do not leave bit patterns for invalid value. For instance a 32-bit integer will likely valid under any circumstances.