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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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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.
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These aspects are inherited by descendants of T (including the class-wide type T'Class).
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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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  The Constant_Indexing and Variable_Indexing aspects are nonoverridable (see 13.1.1).
Paragraphs 6 through 9 were deleted. 

Syntax

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generalized_indexing ::= indexable_container_object_prefix actual_parameter_part

Name Resolution Rules

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 The expected type for the indexable_container_object_prefix of a generalized_indexing is any indexable container type.
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 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:
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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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when the generalized_indexing is used within a primary where a name denoting a constant is permitted.
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 Otherwise, the generalized_indexing is interpreted as a variable indexing.
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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.
NOTES
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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

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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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IB: aliased Indexed_Barrel;
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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
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