Re: Streams and abnormality

[Adam Beneschan <adam@irvine.com>]

On Tuesday, June 12, 2012 2:31:17 AM UTC-7, Simon Wright wrote:
> Using this code with GCC 4.6, 4.7 and GNAT GPL 2011,
> 
>       Str : aliased {a child of Root_Stream_Type};
>       subtype Short_Int is Integer range 0 .. 42;
>       S : Short_Int;
>    begin
>       Integer'Output (Str'Access, -1);
>       S := Short_Int'Input (Str'Access);
>       Put_Line (S'Img & " " & S'Valid'Img);
> 
> I get the output " 63 FALSE".
> 
>  My reading of AARM05 13.9.1(4..6) [1] is that - because S is a scalar
>  object - S should not have been allowed to become abnormal; I think
>  that, instead, some exception (Constraint_Error? Data_Error?) should
>  have been raised.
> 
> [1] http://www.adaic.org/resources/add_content/standards/05aarm/html/AA-13-9-1.html

You're right; it should have raised Constraint_Error, but I think it has nothing to do with 13.9.1, because Short_Int'Input is defined to return a value of type Short_Int'Base, not Short_Int (13.13.2(23), which actually says Short_Int'Input returns the *type* T of which Short_Int is a subtype, but I think that's the same thing).  Thus, in a normal situation, the result of Short_Int'Input *cannot* be an invalid value, because (if Integer'Size = 16) then 16 bits will be read from the stream, and all possible 16-bit bit patterns are valid values of Short_Int'Base.  I don't see anything in 13.13.2 that says the function result of ST'Input is checked against the constraints defined for subtype ST.  The Constraint_Error should occur during the assignment, when the value of the expression (of type S'Base) needs to be constraint-checked before assigning into S, just as it would if *any* function returning Integer appeared on the right side of the assignment. 

After rereading 13.9.1, though, I think it's wrong to suppose that there's a dichotomy between "abnormal" and "must raise an exception".  13.9.1 appears to define two different concepts, abnormal objects and objects with invalid representation.  A scalar object can have invalid representation without being abnormal, and this condition could be produced by stream operations, such as T'Input where T is an enumeration type.  It looks like the main difference between "abnormal objects" and "scalars with invalid representation" is that the latter is a "bounded error" and there is more definition about what programs are required to do with scalars to ensure they don't make a huge mess.  But raising an exception isn't required.

                               -- Adam