C.5 Aspect Discard_Names
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[Specifying the aspect Discard_Names can be used to request a reduction
in storage used for the names of entities with runtime name text.]
Static Semantics
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An entity with
runtime name text is a nonderived enumeration first
subtype, a tagged first subtype, or an exception.
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For an entity with runtime name text, the following language-defined
representation aspect may be specified:
Discard_Names
The type of aspect Discard_Names is Boolean. If directly specified, the
aspect_definition
shall be a static expression. If not specified (including by inheritance),
the aspect is False.
Aspect Description for Discard_Names:
Requests a reduction in storage for names associated with an entity.
Syntax
The form of a
pragma
Discard_Names is as follows:
Legality Rules
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The
local_name
(if present) shall denote an entity with runtime name text. The pragma
specifies that the aspect Discard_Names for the type or exception has
the value True. Without a
local_name,
the pragma specifies that all entities with runtime name text declared
after the pragma, within the same declarative region have the value True
for aspect Discard_Names. Alternatively, the pragma can be used as a
configuration pragma. If the configuration pragma Discard_Names applies
to a compilation unit, all entities with runtime name text declared in
the compilation unit have the value True for the aspect Discard_Names..
Ramification: {
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If the aspect is specified for a type, then it is inherited by all descendants
of the type. The aspect cannot be specified as False on a derived type
(because specifying the aspect is not allowed on derived enumeration
types, and by rule applying to all aspects for other types (see
13.1.1)).
Static Semantics
If a
local_name
is given, then a
pragma
Discard_Names is a representation pragma.
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If the aspect Discard_Names is True for an enumeration type, then the
semantics of the Wide_Wide_Image and Wide_Wide_Value attributes are implementation
defined for that type[; the semantics of Image, Wide_Image, Value, and
Wide_Value are still defined in terms of Wide_Wide_Image and Wide_Wide_Value].
In addition, the semantics of Text_IO.Enumeration_IO are implementation
defined. If the aspect Discard_Names is True for a tagged type, then
the semantics of the Tags.Wide_Wide_Expanded_Name function are implementation
defined for that type[; the semantics of Tags.Expanded_Name and Tags.Wide_Expanded_Name
are still defined in terms of Tags.Wide_Wide_Expanded_Name]. If the aspect
Discard_Names is True for an exception, then the semantics of the Exceptions.Wide_Wide_Exception_Name
function are implementation defined for that exception[; the semantics
of Exceptions.Exception_Name and Exceptions.Wide_Exception_Name are still
defined in terms of Exceptions.Wide_Wide_Exception_Name].
Implementation defined: The semantics
of some attributes and functions of an entity for which aspect Discard_Names
is True.
Ramification: The Width attribute is
still defined in terms of Image.
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The semantics of S'Wide_Wide_Image and S'Wide_Wide_Value are implementation
defined for any subtype of an enumeration type for which the aspect is
True. (The pragma, if used, actually names the first subtype, of course.)
Implementation Advice
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If the aspect Discard_Names is True for an entity, then the implementation
should reduce the amount of storage used for storing names associated
with that entity.
Implementation Advice: If aspect Discard_Names
is True for an entity, then the amount of storage used for storing names
associated with that entity should be reduced.
Reason: A typical implementation of the
Image attribute for enumeration types is to store a table containing
the names of all the enumeration literals. Aspect Discard_Names allows
the implementation to avoid storing such a table without having to prove
that the Image attribute is never used (which can be difficult in the
presence of separate compilation).
We did not specify the semantics of the Image
attribute when aspect Discard_Names is True because different semantics
might be desirable in different situations. In some cases, it might make
sense to use the Image attribute to print out a useful value that can
be used to identify the entity given information in compiler-generated
listings. In other cases, it might make sense to get an error at compile
time or at run time. In cases where memory is plentiful, the simplest
implementation makes sense: ignore the aspect. Implementations that are
capable of avoiding the extra storage in cases where the Image attribute
is never used might also wish to ignore the aspect.
The same applies to the Tags.Expanded_Name and
Exceptions.Exception_Name functions.
Wording Changes from Ada 95
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Updated the wording to reflect that the double wide image and value functions
are now the master versions that the others are defined from.
Wording Changes from Ada 2012
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Corrigendum: Defined the pragma in terms of the aspect Discard_Names,
and added a missing definition of the meaning of the configuration pragma.
This is not intended to make any semantic change (Ada 2012 has an aspect
Discard_Names defined via blanket rules for representation pragmas in
13.1 and
13.1.1),
just to clarify the meaning.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
