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5.5.1 User-Defined Iterator Types

Static Semantics

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{AI05-0139-2} The following language-defined generic library package exists: 
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generic
   type Cursor;
   with function Has_Element (Position : Cursor) return Boolean;
package Ada.Iterator_Interfaces is
   pragma Pure (Iterator_Interfaces);
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   type Forward_Iterator is limited interface;
   function First (Object : Forward_Iterator) return Cursor is abstract;
   function Next (Object : Forward_Iterator; Position : Cursor)
      return Cursor is abstract;
4/3
   type Reversible_Iterator is limited interface and Forward_Iterator;
   function Last (Object : Reversible_Iterator) return Cursor is abstract;
   function Previous (Object : Reversible_Iterator; Position : Cursor)
      return Cursor is abstract;
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end Ada.Iterator_Interfaces;
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{AI05-0139-2} An iterator type is a type descended from the Forward_Iterator interface from some instance of Ada.Iterator_Interfaces. A reversible iterator type is a type descended from the Reversible_Iterator interface from some instance of Ada.Iterator_Interfaces. An iterator object is an object of an iterator type. A reversible iterator object is an object of a reversible iterator type. The formal subtype Cursor from the associated instance of Ada.Iterator_Interfaces is the iteration cursor subtype for the iterator type.
7/3
{AI05-0139-2} {AI05-0292-1} The following type-related operational aspects may be specified for an indexable container type T (see 4.1.6):
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Default_Iterator

This aspect is specified by a name that denotes exactly one function declared immediately within the same declaration list in which T is declared, whose first parameter is of type T or T'Class or an access parameter whose designated type is type T or T'Class, whose other parameters, if any, have default expressions, and whose result type is an iterator type. This function is the default iterator function for T. Its result subtype is the default iterator subtype for T. The iteration cursor subtype for the default iterator subtype is the default cursor subtype for T.
8.a/3
Aspect Description for Default_Iterator: Default iterator to be used in for loops.
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Iterator_Element

This aspect is specified by a name that denotes a subtype. This is the default element subtype for T.
9.a/3
Aspect Description for Iterator_Element: Element type to be used for user-defined iterators.
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These aspects are inherited by descendants of type T (including T'Class).
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{AI05-0139-2} {AI05-0292-1} An iterable container type is an indexable container type with specified Default_Iterator and Iterator_Element aspects. A reversible iterable container type is an iterable container type with the default iterator type being a reversible iterator type. An iterable container object is an object of an iterable container type. A reversible iterable container object is an object of a reversible iterable container type.
11.a.1/3
Glossary entry: An iterable container type is one that has user-defined behavior for iteration, via the Default_Iterator and Iterator_Element aspects.
11.1/4
  {AI12-0138-1} The Default_Iterator and Iterator_Element aspects are nonoverridable (see 13.1.1).
11.a/4
Reason: This ensures that all descendants of an iterable container type have aspects with the same properties. This prevents generic contract problems with formal derived types. 

Legality Rules

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{AI05-0139-2} {AI05-0292-1} The Constant_Indexing aspect (if any) of an iterable container type T shall denote exactly one function with the following properties:
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the result type of the function is covered by the default element type of T or is a reference type (see 4.1.5) with an access discriminant designating a type covered by the default element type of T;
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the type of the second parameter of the function covers the default cursor type for T;
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if there are more than two parameters, the additional parameters all have default expressions.
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This function (if any) is the default constant indexing function for T.
16.a/3
Ramification: This does not mean that Constant_Indexing has to designate only one subprogram, only that there is only one routine that meets all of these properties. There can be other routines designated by Constant_Indexing, but they cannot have the profile described above. For instance, map containers have a version of Constant_Indexing that takes a key instead of a cursor; this is allowed.
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{AI05-0139-2} {AI05-0292-1} The Variable_Indexing aspect (if any) of an iterable container type T shall denote exactly one function with the following properties:
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the result type of the function is a reference type (see 4.1.5) with an access discriminant designating a type covered by the default element type of T;
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the type of the second parameter of the function covers the default cursor type for T;
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if there are more than two parameters, the additional parameters all have default expressions.
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This function (if any) is the default variable indexing function for T.

Extensions to Ada 2005

21.a/3
{AI05-0139-2} User-defined iterator types are new in Ada 2012.

Incompatibilities With Ada 2012

21.b/4
{AI12-0138-1} Corrigendum: Defined Default_Iterator and Iterator_Element to be nonoveridable, which makes redefinitions and hiding of these aspects illegal. It's possible that some program could violate one of these new restrictions, but in most cases this can easily be worked around by using overriding rather than redefinition. 

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