Version 1.3 of ais/ai-00108.txt

Unformatted version of ais/ai-00108.txt version 1.3
Other versions for file ais/ai-00108.txt

!standard 13.13.02 (09)          99-08-31 AI95-00108/03
!standard 13.13.02 (27)
!standard 13.13.02 (36)
!class binding interpretation 96-04-04
!status Corrigendum 2000 99-06-02
!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
!qualifier Error
!subject Inheritance of Stream Attributes for Type Extensions
!summary
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
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
(See summary.)
!wording
(See corrigendum.)
!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 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.
!corrigendum 13.13.02(9)
Replace the paragraph:
For elementary types, the representation in terms of stream elements is implementation defined. For composite types, the Write or Read attribute for each component is called in a canonical order. The canonical order of components is last dimension varying fastest for an array, and positional aggregate order for a record. Bounds are not included in the stream if T is an array type. If T is a discriminated type, discriminants are included only if they have defaults. If T is a tagged type, the tag is not included.
by:
For elementary types, the representation in terms of stream elements is implementation defined. For composite types, the Write or Read attribute for each component is called in a canonical order. The canonical order of components is last dimension varying fastest for an array, and positional aggregate order for a record. Bounds are not included in the stream if T is an array type. If T is a discriminated type, discriminants are included only if they have defaults. If T is a tagged type, the tag is not included. For nonlimited type extensions, the Write or Read attribute for the parent type is called, followed by the Write or Read attribute of each non-inherited component, in canonical order. For limited derived types, the Write or Read attribute for the parent type is called.
!corrigendum 13.13.02(27)
Insert after the paragraph:
the new paragraph:
If T is a derived type, S'Output and S'Input are not inherited from the parent type; they are defined in terms of S'Read and S'Write, notwithstanding the inheritance rule stated in 13.1.
!corrigendum 13.13.02(36)
Insert after the paragraph:
The stream-oriented attributes may be specified for any type via an attribute_definition_clause. All nonlimited types have default implementations for these operations. An attribute_reference for one of these attributes is illegal if the type is limited, unless the attribute has been specified by an attribute_definition_clause. For an attribute_definition_clause specifying one of these attributes, the subtype of the Item parameter shall be the base subtype if scalar, and the first subtype otherwise. The same rule applies to the result of the Input function.
the new paragraphs:
For every subtype S of a language-defined nonlimited specific type T, the output generated by S'Output or S'Write must be readable by S'Input or S'Read, respectively. The object read by S'Input or S'Read must behave as the original object for the operations declared in the predefined descendants of the unit that declares T. This rule applies across partitions if the implementation conforms to the Distributed Systems Annex.
For every subtype S'Class of a language-defined class-wide type T'Class, the attributes S'Class'Write, S'Class'Read, S'Class'Input and S'Class'Output must have their predefined definition.
!ACATS test
A C-Test to check that the Read and Write attributes of an extension are defined as in this issue should be created. It should also check that the Input and Output attributes revert to the predefined versions for an extension. Tests to check that language defined nonlimited private types properly support 'Read and 'Write should also be created.
!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