Ada 2012 Reference Manual
4.1.6 User-Defined Indexing
Static Semantics
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.
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.
These aspects are inherited by descendants of T (including the class-wide type T'Class).
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.
The Constant_Indexing and Variable_Indexing aspects are nonoverridable (see 13.1.1).
Paragraphs 6 through 9 were deleted.
Syntax
Name Resolution Rules
The expected type for the indexable_container_object_prefix of a generalized_indexing is any indexable container type.
If the Constant_Indexing aspect is specified for the type of the indexable_container_object_prefix of a generalized_indexing, then the generalized_indexing is interpreted as a constant indexing under the following circumstances:
- 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;
- when the generalized_indexing is used within a primary where a name denoting a constant is permitted.
Otherwise, the generalized_indexing is interpreted as a variable indexing.
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.
NOTES
6 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
type Indexed_Barrel is tagged ...
with Variable_Indexing => Find;
-- Indexed_Barrel is an indexable container type,
-- Find is the generalized indexing operation.
with Variable_Indexing => Find;
-- Indexed_Barrel is an indexable container type,
-- Find is the generalized indexing operation.
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).
-- Return a reference to an element of a barrel (see 4.1.5).
IB: aliased Indexed_Barrel;
-- 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
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
