4.1.6 User-Defined Indexing
Static Semantics
{
AI05-0139-2}
Given a tagged type
T, the following type-related, operational
aspects may be specified:
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.
Aspect Description for Constant_Indexing:
Defines function(s) to implement user-defined
indexed_components.
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.
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.
Aspect Description for Variable_Indexing:
Defines function(s) to implement user-defined
indexed_components.
{
AI12-0104-1}
These aspects are inherited by descendants of
T (including the
class-wide type
T'Class).
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.
{
AI05-0139-2}
{
AI05-0292-1}
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.]
Glossary entry: An indexable container
type is one that has user-defined behavior for indexing, via the Constant_Indexing
or Variable_Indexing aspects.
{
AI12-0138-1}
The Constant_Indexing and Variable_Indexing aspects are nonoverridable
(see
13.1.1).
Reason: This ensures that all descendants
of an indexable container type have aspects with the same properties.
This prevents generic contract problems with formal derived types.
{
AI12-0104-1}
{
AI12-0138-1}
A nonoverridable aspect allows the replacement of the implementation
of an indexing function and the addition of a new indexing function for
a derived type, but not the removal of an indexing function. This is
necessary so that indexing can be used on objects of T'Class. So long
as the tag of O is that of its nominal subtype, we do not want T'Class(O)(I)
to mean something different than O(I). Thus we cannot allow a change
in the function identified. As T'Class(O)(I) expands into a dispatching
call, we need to ensure that there is a body for each such function --
but it is OK for that body to be changed from the original body (that's
just normal dispatching).
Paragraphs 6 through
9 were deleted.
Syntax
Name Resolution Rules
when the Variable_Indexing aspect is not specified
for the type of the
indexable_container_object_prefix;
when the
indexable_container_object_prefix
denotes a constant;
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.
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.
{
AI12-0005-1}
where Indexing is the
name
specified for the Constant_Indexing or Variable_Indexing aspect. This
equivalence is then resolved in the normal way; the aspect specifies
a
name, it
does not denote declarations.
6 {
AI12-0104-1}
The Constant_Indexing and Variable_Indexing aspects cannot be redefined
when inherited for a derived type, but the functions that they denote
can be modified by overriding or overloading.
Examples
{
AI05-0268-1}
{
AI05-0292-1}
type Indexed_Barrel
is tagged ...
with Variable_Indexing => Find;
--
Indexed_Barrel is an indexable container type,
--
Find is the generalized indexing operation.{
AI05-0268-1}
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).{
AI05-0268-1}
--
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 onlyExtensions to Ada 2005
Wording Changes from Ada 2012
{
AI12-0104-1}
Corrigendum: Converted confusing and unnecessary normative wording
about "overriding an aspect" into a note.
{
AI12-0138-1}
Corrigendum: Defined Constant_Indexing and Variable_Indexing to
be nonoveridable. This is merely a new description for Legality Rules
which already applied to these aspects.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
