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5.5.1 User-Defined Iterator Types

Static Semantics

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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;
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   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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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.
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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.
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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.
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 These aspects are inherited by descendants of type T (including T'Class).
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 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.
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   The Default_Iterator and Iterator_Element aspects are nonoverridable (see 13.1.1).

Legality Rules

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 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.
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 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.

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