5.5.1 User-Defined Iterator Types
Static Semantics
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The following language-defined generic library package exists:
generic
type Cursor;
with function Has_Element (Position : Cursor)
return Boolean;
package Ada.Iterator_Interfaces
is
pragma Pure (Iterator_Interfaces);
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;
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;
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):
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.
Aspect Description for Default_Iterator:
Default iterator to be used in for loops.
Iterator_Element
This aspect is specified by a
name
that denotes a subtype. This is the
default element subtype for
T.
Aspect Description for Iterator_Element:
Element type to be used for user-defined iterators.
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.
Glossary entry: An iterable container
type is one that has user-defined behavior for iteration, via the Default_Iterator
and Iterator_Element aspects.
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The Default_Iterator and Iterator_Element aspects are nonoverridable
(see
13.1.1).
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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The Constant_Indexing aspect (if any) of an iterable container type
T
shall denote exactly one function with the following properties:
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;
the type of the second parameter of the function
covers the default cursor type for T;
if there are more than two parameters, the additional
parameters all have default expressions.
This function (if any) is the
default constant
indexing function for
T.
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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The Variable_Indexing aspect (if any) of an iterable container type
T
shall denote exactly one function with the following properties:
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;
the type of the second parameter of the function
covers the default cursor type for T;
if there are more than two parameters, the additional
parameters all have default expressions.
This function (if any) is the
default variable
indexing function for
T.
Extensions to Ada 2005
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User-defined iterator types are new in Ada 2012.
Incompatibilities With Ada 2012
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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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
