Version 1.1.1.1 of ais/ai-00108.txt

Unformatted version of ais/ai-00108.txt version 1.1.1.1
Other versions for file ais/ai-00108.txt

!standard 13.13.02 (09)          96-09-15 AI95-00108/02
!class binding interpretation 96-04-04
!status WG9 approved 96-12-07
!status ARG approved 7-0-2 96-06-17
!status work item 96-04-13
!status received 96-04-04
!priority High
!difficulty Hard
!subject Inheritance of Stream Attributes for Type Extensions
!summary 96-04-04
For a type extension, the predefined Read attribute is defined to call the Read of the parent type, followed by the Read of the non-inherited components, if any, in canonical order. The analogous rule applies to the Write attribute.
The Input and Output attributes are not inherited by a type extension.
The stream attributes must work properly for every language defined nonlimited type. For language defined private types, the output generated by the Write attribute is not specified, but it must be readable by the Read attribute.
!question 96-04-04
13.1(15) says:
15 A derived type inherits each type-related aspect of its parent type that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent type from the grandparent type. A derived subtype inherits each subtype-specific aspect of its parent subtype that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent subtype from the grandparent subtype, but only if the parent subtype statically matches the first subtype of the parent type. An inherited aspect of representation is overridden by a subsequent representation item that specifies the same aspect of the type or subtype.
Do these rules apply to the stream-oriented attributes Read, Write, Input, and Output? (No.)
Are the stream-oriented attributes intended to work properly for language defined types such as Unbounded_String? (Yes.)
!recommendation 96-04-04
(See summary.)
!wording 96-04-04
!discussion 96-07-23
The general rule for inheritance of type-related representation aspects should not apply to the stream attributes of type extensions. For 'Read and 'Write, a rule analogous to the rule for tagged equality makes the most sense. For 'Input and 'Output, no inheritance makes sense; instead, they should regain their predefined meaning in terms of 'Read and 'Write.
There are several problems associated with applying the normal 13.1(15) inheritance rules to the stream attributes of tagged types:
1) Inheriting a 'Read or 'Write of the parent type as-is for
the 'Read or 'Write of a type extension will ignore any new components added in the extension part. A rule analogous to the one for the equality operator makes more sense. In particular, the default 'Read or 'Write for a type extension should be defined to do the 'Read or 'Write of the parent type followed by the 'Read or 'Write for each component of the type extension, in canonical order.
2) Inheriting a 'Input or 'Output of the parent type as-is for
'Input or 'Output of a type extension makes no sense, since the inherited 'Input is a function returning the parent type, and the inherited 'Output puts out the discriminants of the parent type. For these two, the only meaningful approach seems to be for the default 'Input and 'Output for a tagged type to always be defined in terms of the 'Read and 'Write for the tagged type, preceded with the discriminants, if any.
For untagged derived types, there is no problem for the derived type inheriting the stream attributes. Even for tagged derived types, if the extension part is null, the 'Read and 'Write will effectively be inherited.
For language defined nonlimited private types, the RM does not say whether the stream-oriented attributes must work properly. It seems that they ought to. For many such types, the default version will work properly. However, for a type like Unbounded_String, which is almost certainly implemented as a data structure involving pointers, the default versions will not work. Therefore, for these types, the implementer must provide an explicit version of the Read and Write attributes.
!appendix

!section 13.13.2(09)
!subject Missing AI: 'Input/'Output inheritance
!reference RM95-13.13.2(9,36)
!reference RM95-13.1(15)
!from Tucker Taft 95-10-30
!reference 95-5373.a Tucker Taft 95-10-30>>
!discussion

The general rule for inheritance of type-related representation aspects
should not apply to the stream attributes of type extensions.  
For 'Read and 'Write, a rule analogous to the rule for tagged equality 
would make the most sense.  For 'Input and 'Output, no inheritance makes 
sense; instead, they should regain their predefined meaning in terms
of 'Read and 'Write.

This issue was discussed at length in some Ada 95 forum or other 
(perhaps the GNAT team mailing list?).  There are several problems
associated with applying the normal 13.1(15) inheritance rules to 
the stream attributes of tagged types:

 1) Inheriting a 'Read or 'Write of the parent type as-is for 
    the 'Read or 'Write of a type extension will ignore any new 
    components added in the extension part.  A rule analogous 
    to the one for the equality operator would make more sense.
    In particular, the default 'Read or 'Write for a type extension
    should be defined to do the 'Read or 'Write of the parent type
    followed by the 'Read or 'Write for each component of the
    type extension, in canonical order.

 2) Inheriting a 'Input or 'Output of the parent type as-is for
    'Input or 'Output of a type extension makes no sense, since
    the inherited 'Input is a function returning the parent type,
    and the inherited 'Output puts out the discriminants of the
    parent type.  For these two, the only meaningful approach seems
    to be for the default 'Input and 'Output for a tagged type to
    always be defined in terms of the 'Read and 'Write for the
    tagged type, preceded with the discriminants, if any.

For untagged derived types, there is no problem for the derived
type inheriting the stream attributes.  Even for tagged derived
types, if the extension part is null, the 'Read and 'Write will
effectively be inherited, given the rule suggested above.

****************************************************************

!section 13.3.2(00)
!subject stream attributes for unbounded string, bounded string etc.
!reference RM95-13.3.2(00)
!from Bob Duff
!reference 96-5480.a Robert A Duff 96-4-12>>
!discussion

Robert Dewar pointed out this issue to me.

It would seem that the stream attributes should work properly for all
the language-defined non-limited types.  For example, types
Strings.Unbounded.Unbounded_String, Strings.Bounded.Bounded_String, etc.

- Bob

****************************************************************

!section 13.13.2(00)
!subject Stream-Oriented attributes for language-defined private types
!reference RM95-13.13.2
!from Laurent Guerby 96-07-15
!reference 96-5621.a Laurent Guerby  96-7-15>>
!discussion

   The behaviour of the stream oriented attributes is not clearly
defined for most of private (or implementation-defined) types. This
can lead to erroneous execution (but nothing is said about it in the
RM95) if type type has a component of an access type when some object
of this type is exchanged between two partitions. An example of
interest is Unbounded_String.

--
Laurent Guerby <guerby@gnat.com>, Team Ada.
   "Use the Source, Luke. The Source will be with you, always (GPL)."

****************************************************************

Questions? Ask the ACAA Technical Agent