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< title > Indirect Iterator</ title >
< meta name = "author" content = "David Abrahams, Jeremy Siek, Thomas Witt" />
< meta name = "organization" content = "Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
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< body >
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< div class = "document" id = "indirect-iterator" >
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< h1 class = "title" > Indirect Iterator</ h1 >
< table class = "docinfo" frame = "void" rules = "none" >
< col class = "docinfo-name" />
< col class = "docinfo-content" />
< tbody valign = "top" >
< tr >< th class = "docinfo-name" > Author:</ th >
< td > David Abrahams, Jeremy Siek, Thomas Witt</ td ></ tr >
< tr >< th class = "docinfo-name" > Contact:</ th >
< td >< a class = "first reference" href = "mailto:dave@boost-consulting.com" > dave@ boost-consulting.com</ a > , < a class = "reference" href = "mailto:jsiek@osl.iu.edu" > jsiek@ osl.iu.edu</ a > , < a class = "last reference" href = "mailto:witt@ive.uni-hannover.de" > witt@ ive.uni-hannover.de</ a ></ td ></ tr >
< tr >< th class = "docinfo-name" > Organization:</ th >
< td >< a class = "first reference" href = "http://www.boost-consulting.com" > Boost Consulting</ a > , Indiana University < a class = "reference" href = "http://www.osl.iu.edu" > Open Systems
Lab</ a > , University of Hanover < a class = "last reference" href = "http://www.ive.uni-hannover.de" > Institute for Transport
Railway Operation and Construction</ a ></ td ></ tr >
< tr >< th class = "docinfo-name" > Date:</ th >
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< td > 2004-11-01</ td ></ tr >
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< tr >< th class = "docinfo-name" > Copyright:</ th >
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< td > Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</ td ></ tr >
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</ tbody >
</ table >
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<!-- Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > abstract:</ th >< td class = "field-body" > <!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > adapts an iterator by applying an
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< em > extra</ em > dereference inside of < tt class = "docutils literal" >< span class = "pre" > operator*()</ span ></ tt > . For example, this
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iterator adaptor makes it possible to view a container of pointers
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(e.g. < tt class = "docutils literal" >< span class = "pre" > list< foo*> </ span ></ tt > ) as if it were a container of the pointed-to type
(e.g. < tt class = "docutils literal" >< span class = "pre" > list< foo> </ span ></ tt > ). < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > depends on two
auxiliary traits, < tt class = "docutils literal" >< span class = "pre" > pointee</ span ></ tt > and < tt class = "docutils literal" >< span class = "pre" > indirect_reference</ span ></ tt > , to
provide support for underlying iterators whose < tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > is
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not an iterator.</ td >
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</ tr >
</ tbody >
</ table >
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< div class = "contents topic" >
< p class = "topic-title first" >< a id = "table-of-contents" name = "table-of-contents" > Table of Contents</ a ></ p >
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< ul class = "simple" >
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< li >< a class = "reference" href = "#indirect-iterator-synopsis" id = "id2" name = "id2" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > synopsis</ a ></ li >
< li >< a class = "reference" href = "#indirect-iterator-requirements" id = "id3" name = "id3" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > requirements</ a ></ li >
< li >< a class = "reference" href = "#indirect-iterator-models" id = "id4" name = "id4" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > models</ a ></ li >
< li >< a class = "reference" href = "#indirect-iterator-operations" id = "id5" name = "id5" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > operations</ a ></ li >
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< li >< a class = "reference" href = "#example" id = "id6" name = "id6" > Example</ a ></ li >
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</ ul >
</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id2" id = "indirect-iterator-synopsis" name = "indirect-iterator-synopsis" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > synopsis</ a ></ h1 >
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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< pre class = "literal-block" >
template <
class Iterator
, class Value = use_default
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, class CategoryOrTraversal = use_default
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, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator
{
public:
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typedef /* see below */ value_type;
typedef /* see below */ reference;
typedef /* see below */ pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
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indirect_iterator();
indirect_iterator(Iterator x);
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template <
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class Iterator2, class Value2, class Category2
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, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
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Iterator2, Value2, Category2, Reference2, Difference2
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> const& y
, typename enable_if_convertible< Iterator2, Iterator> ::type* = 0 // exposition
);
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Iterator const& base() const;
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reference operator*() const;
indirect_iterator& operator++();
indirect_iterator& operator--();
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private:
Iterator m_iterator; // exposition
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};
</ pre >
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< p > The member types of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > are defined according to
the following pseudo-code, where < tt class = "docutils literal" >< span class = "pre" > V</ span ></ tt > is
< tt class = "docutils literal" >< span class = "pre" > iterator_traits< Iterator> ::value_type</ span ></ tt ></ p >
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< pre class = "literal-block" >
if (Value is use_default) then
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typedef remove_const< pointee< V> ::type> ::type value_type;
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else
typedef remove_const< Value> ::type value_type;
if (Reference is use_default) then
if (Value is use_default) then
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typedef indirect_reference< V> ::type reference;
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else
typedef Value& reference;
else
typedef Reference reference;
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if (Value is use_default) then
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typedef pointee< V> ::type* pointer;
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else
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typedef Value* pointer;
if (Difference is use_default)
typedef iterator_traits< Iterator> ::difference_type difference_type;
else
typedef Difference difference_type;
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if (CategoryOrTraversal is use_default)
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typedef < em > iterator-category</ em > (
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iterator_traversal< Iterator> ::type,``reference``,``value_type``
) iterator_category;
else
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typedef < em > iterator-category</ em > (
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CategoryOrTraversal,``reference``,``value_type``
) iterator_category;
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</ pre >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id3" id = "indirect-iterator-requirements" name = "indirect-iterator-requirements" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > requirements</ a ></ h1 >
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< p > The expression < tt class = "docutils literal" >< span class = "pre" > *v</ span ></ tt > , where < tt class = "docutils literal" >< span class = "pre" > v</ span ></ tt > is an object of
< tt class = "docutils literal" >< span class = "pre" > iterator_traits< Iterator> ::value_type</ span ></ tt > , shall be valid
expression and convertible to < tt class = "docutils literal" >< span class = "pre" > reference</ span ></ tt > . < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > shall
model the traversal concept indicated by < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > .
< tt class = "docutils literal" >< span class = "pre" > Value</ span ></ tt > , < tt class = "docutils literal" >< span class = "pre" > Reference</ span ></ tt > , and < tt class = "docutils literal" >< span class = "pre" > Difference</ span ></ tt > shall be chosen so
that < tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > , < tt class = "docutils literal" >< span class = "pre" > reference</ span ></ tt > , and < tt class = "docutils literal" >< span class = "pre" > difference_type</ span ></ tt > meet
the requirements indicated by < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > .</ p >
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< p > [Note: there are further requirements on the
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< tt class = "docutils literal" >< span class = "pre" > iterator_traits< Iterator> ::value_type</ span ></ tt > if the < tt class = "docutils literal" >< span class = "pre" > Value</ span ></ tt >
parameter is not < tt class = "docutils literal" >< span class = "pre" > use_default</ span ></ tt > , as implied by the algorithm for
deducing the default for the < tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > member.]</ p >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id4" id = "indirect-iterator-models" name = "indirect-iterator-models" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > models</ a ></ h1 >
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< p > In addition to the concepts indicated by < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt >
and by < tt class = "docutils literal" >< span class = "pre" > iterator_traversal< indirect_iterator> ::type</ span ></ tt > , a
specialization of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > models the following
concepts, Where < tt class = "docutils literal" >< span class = "pre" > v</ span ></ tt > is an object of
< tt class = "docutils literal" >< span class = "pre" > iterator_traits< Iterator> ::value_type</ span ></ tt > :</ p >
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< blockquote >
< ul class = "simple" >
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< li > Readable Iterator if < tt class = "docutils literal" >< span class = "pre" > reference(*v)</ span ></ tt > is convertible to
< tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > .</ li >
< li > Writable Iterator if < tt class = "docutils literal" >< span class = "pre" > reference(*v)</ span > < span class = "pre" > =</ span > < span class = "pre" > t</ span ></ tt > is a valid
expression (where < tt class = "docutils literal" >< span class = "pre" > t</ span ></ tt > is an object of type
< tt class = "docutils literal" >< span class = "pre" > indirect_iterator::value_type</ span ></ tt > )</ li >
< li > Lvalue Iterator if < tt class = "docutils literal" >< span class = "pre" > reference</ span ></ tt > is a reference type.</ li >
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</ ul >
</ blockquote >
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< p >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator< X,V1,C1,R1,D1> </ span ></ tt > is interoperable with
< tt class = "docutils literal" >< span class = "pre" > indirect_iterator< Y,V2,C2,R2,D2> </ span ></ tt > if and only if < tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt > is
interoperable with < tt class = "docutils literal" >< span class = "pre" > Y</ span ></ tt > .</ p >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id5" id = "indirect-iterator-operations" name = "indirect-iterator-operations" >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > operations</ a ></ h1 >
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< p > In addition to the operations required by the concepts described
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above, specializations of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > provide the
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following operations.</ p >
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< p >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator();</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Requires:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > must be Default Constructible.</ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Effects:</ th >< td class = "field-body" > Constructs an instance of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > with
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a default-constructed < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator(Iterator</ span > < span class = "pre" > x);</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Effects:</ th >< td class = "field-body" > Constructs an instance of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > with
< tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > copy constructed from < tt class = "docutils literal" >< span class = "pre" > x</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
< pre class = "literal-block" >
template <
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class Iterator2, class Value2, unsigned Access, class Traversal
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, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
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Iterator2, Value2, Access, Traversal, Reference2, Difference2
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> const& y
, typename enable_if_convertible< Iterator2, Iterator> ::type* = 0 // exposition
);
</ pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Requires:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > Iterator2</ span ></ tt > is implicitly convertible to < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > .</ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Effects:</ th >< td class = "field-body" > Constructs an instance of < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > whose
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< tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > subobject is constructed from < tt class = "docutils literal" >< span class = "pre" > y.base()</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > Iterator</ span > < span class = "pre" > const& </ span > < span class = "pre" > base()</ span > < span class = "pre" > const;</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > reference</ span > < span class = "pre" > operator*()</ span > < span class = "pre" > const;</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > **m_iterator</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator& </ span > < span class = "pre" > operator++();</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Effects:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > ++m_iterator</ span ></ tt ></ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > *this</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > indirect_iterator& </ span > < span class = "pre" > operator--();</ span ></ tt ></ p >
< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Effects:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > --m_iterator</ span ></ tt ></ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Returns:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > *this</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id6" id = "example" name = "example" > Example</ a ></ h1 >
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< p > This example prints an array of characters, using
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< tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > to access the array of characters through an
array of pointers. Next < tt class = "docutils literal" >< span class = "pre" > indirect_iterator</ span ></ tt > is used with the
< tt class = "docutils literal" >< span class = "pre" > transform</ span ></ tt > algorithm to copy the characters (incremented by one) to
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another array. A constant indirect iterator is used for the source and
a mutable indirect iterator is used for the destination. The last part
of the example prints the original array of characters, but this time
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using the < tt class = "docutils literal" >< span class = "pre" > make_indirect_iterator</ span ></ tt > helper function.</ p >
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< pre class = "literal-block" >
char characters[] = " abcdefg" ;
const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char
char* pointers_to_chars[N]; // at the end.
for (int i = 0; i < N; ++i)
pointers_to_chars[i] = & characters[i];
// Example of using indirect_iterator
boost::indirect_iterator< char**, char>
indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N);
std::copy(indirect_first, indirect_last, std::ostream_iterator< char> (std::cout, " ," ));
std::cout << std::endl;
// Example of making mutable and constant indirect iterators
char mutable_characters[N];
char* pointers_to_mutable_chars[N];
for (int j = 0; j < N; ++j)
pointers_to_mutable_chars[j] = & mutable_characters[j];
boost::indirect_iterator< char* const*> mutable_indirect_first(pointers_to_mutable_chars),
mutable_indirect_last(pointers_to_mutable_chars + N);
boost::indirect_iterator< char* const*, char const> const_indirect_first(pointers_to_chars),
const_indirect_last(pointers_to_chars + N);
std::transform(const_indirect_first, const_indirect_last,
mutable_indirect_first, std::bind1st(std::plus< char> (), 1));
std::copy(mutable_indirect_first, mutable_indirect_last,
std::ostream_iterator< char> (std::cout, " ," ));
std::cout << std::endl;
// Example of using make_indirect_iterator()
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std::copy(boost::make_indirect_iterator(pointers_to_chars),
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boost::make_indirect_iterator(pointers_to_chars + N),
std::ostream_iterator< char> (std::cout, " ," ));
std::cout << std::endl;
</ pre >
< p > The output is:</ p >
< pre class = "literal-block" >
a,b,c,d,e,f,g,
b,c,d,e,f,g,h,
a,b,c,d,e,f,g,
</ pre >
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< p > The source code for this example can be found < a class = "reference" href = "../example/indirect_iterator_example.cpp" > here</ a > .</ p >
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</ div >
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</ div >
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</ body >
</ html >
