4.1.6 User-Defined Indexing
Static Semantics
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Given a tagged type
T, the following type-related, operational
aspects may be specified:
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Constant_Indexing
This aspect shall be specified by a
name
that denotes one or more functions declared immediately within the same
declaration list in which
T is declared. All such functions shall
have at least two parameters, the first of which is of type
T
or
T'Class, or is an access-to-constant parameter with designated
type
T or
T'Class.
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Aspect Description for Constant_Indexing:
Defines function(s) to implement user-defined
indexed_components.
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Variable_Indexing
This aspect shall be specified by a
name
that denotes one or more functions declared immediately within the same
declaration list in which
T is declared. All such functions shall
have at least two parameters, the first of which is of type
T
or
T'Class, or is an access parameter with designated type
T
or
T'Class. All such functions shall have a return type that is
a reference type (see
4.1.5), whose reference
discriminant is of an access-to-variable type.
3.a/3
Reason: We require these functions to
return a reference type so that the object returned from the function
can act like a variable. We need no similar rule for Constant_Indexing,
since all functions return constant objects.
3.b/3
Aspect Description for Variable_Indexing:
Defines function(s) to implement user-defined
indexed_components.
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These aspects are inherited by descendants of T
(including the class-wide type T'Class). [The aspects shall not
be overridden, but the functions they denote may be.]
4.a/3
Ramification: Indexing can be provided
for multiple index types by overloading routines with different parameter
profiles. For instance, the map containers provide indexing on both cursors
and keys by providing pairs of overloaded routines to the Constant_Indexing
and Variable_Indexing aspects.
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An
indexable container type is (a view of) a tagged type with
at least one of the aspects Constant_Indexing or Variable_Indexing specified.
An
indexable container object is an object of an indexable container
type.
[A
generalized_indexing
is a
name
that denotes the result of calling a function named by a Constant_Indexing
or Variable_Indexing aspect.]
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Glossary entry: An indexable container
type is one that has user-defined behavior for indexing, via the Constant_Indexing
or Variable_Indexing aspects.
Legality Rules
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The Constant_Indexing or Variable_Indexing aspect shall not be specified:
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on a derived type if the parent type has the corresponding
aspect specified or inherited; or
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9/3
In addition to the places where
Legality Rules normally apply (see
12.3),
these rules apply also in the private part of an instance of a generic
unit.
9.a/3
Ramification: In order to enforce these
rules without breaking privacy, we cannot allow a tagged private type
to have hidden indexing aspects. There is no problem if the private type
is not tagged (as the indexing aspects cannot be specified on descendants
in that case).
9.b/3
We don't need an assume-the-worst rule as deriving
from formal tagged type is not allowed in generic bodies.
Syntax
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Name Resolution Rules
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when the Variable_Indexing aspect is not specified
for the type of the
indexable_container_object_prefix;
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when the
indexable_container_object_prefix
denotes a constant;
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15.a/3
Ramification: This means it is not interpreted
as a constant indexing for the
variable_name
in the LHS of an assignment (not inside a
primary),
nor for the
name
used for an
out or
in out parameter (not allowed to be
a constant), nor for the
name
in an object renaming (not inside a primary), unless there is no Variable_Indexing
aspect defined.
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When a
generalized_indexing
is interpreted as a constant (or variable) indexing, it is equivalent
to a call on a prefixed view of one of the functions named by the Constant_Indexing
(or Variable_Indexing) aspect of the type of the
indexable_container_object_prefix
with the given
actual_parameter_part,
and with the
indexable_container_object_prefix
as the
prefix
of the prefixed view.
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17.b/3
17.c/3
where Indexing
is the name specified for the Constant_Indexing or Variable_Indexing
aspect.
Examples
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type Indexed_Barrel
is tagged ...
with Variable_Indexing => Find;
--
Indexed_Barrel is an indexable container type,
--
Find is the generalized indexing operation.
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function Find (B :
aliased in out Indexed_Barrel; Key : String)
return Ref_Element;
--
Return a reference to an element of a barrel (see 4.1.5).
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--
All of the following calls are then equivalent:
Find (IB,"pear").Data.
all := Element'(...); --
Traditional call
IB.Find ("pear").Data.
all := Element'(...); --
Call of prefixed view
IB.Find ("pear") := Element'(...); --
Implicit dereference (see 4.1.5)
IB ("pear") := Element'(...); --
Implicit indexing and dereference
IB ("pear").Data.
all := Element'(...); --
Implicit indexing only
Extensions to Ada 2005
21.a/3
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe