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< title > Transform Iterator</ title >
< meta name = "author" content = "David Abrahams, Jeremy Siek, Thomas Witt" />
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< body >
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< div class = "document" id = "transform-iterator" >
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< h1 class = "title" > Transform 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) -->
The transform iterator adapts an iterator by modifying the
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< tt class = "docutils literal" >< span class = "pre" > operator*</ span ></ tt > to apply a function object to the result of
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dereferencing the iterator and returning the result.</ 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 = "#transform-iterator-synopsis" id = "id2" name = "id2" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > synopsis</ a ></ li >
< li >< a class = "reference" href = "#transform-iterator-requirements" id = "id3" name = "id3" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > requirements</ a ></ li >
< li >< a class = "reference" href = "#transform-iterator-models" id = "id4" name = "id4" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models</ a ></ li >
< li >< a class = "reference" href = "#transform-iterator-operations" id = "id5" name = "id5" >< tt class = "docutils literal" >< span class = "pre" > transform_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 = "transform-iterator-synopsis" name = "transform-iterator-synopsis" >< tt class = "docutils literal" >< span class = "pre" > transform_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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<!-- Version 1.3 of this document was accepted for TR1 -->
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< pre class = "literal-block" >
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template < class UnaryFunction,
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class Iterator,
class Reference = use_default,
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class Value = use_default>
class transform_iterator
{
public:
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typedef /* see below */ value_type;
typedef /* see below */ reference;
typedef /* see below */ pointer;
typedef iterator_traits< Iterator> ::difference_type difference_type;
typedef /* see below */ iterator_category;
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transform_iterator();
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transform_iterator(Iterator const& x, UnaryFunction f);
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template< class F2, class I2, class R2, class V2>
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transform_iterator(
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transform_iterator< F2, I2, R2, V2> const& t
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, typename enable_if_convertible< I2, Iterator> ::type* = 0 // exposition only
, typename enable_if_convertible< F2, UnaryFunction> ::type* = 0 // exposition only
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);
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UnaryFunction functor() const;
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Iterator const& base() const;
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reference operator*() const;
transform_iterator& operator++();
transform_iterator& operator--();
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private:
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Iterator m_iterator; // exposition only
UnaryFunction m_f; // exposition only
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};
</ pre >
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< p > If < tt class = "docutils literal" >< span class = "pre" > Reference</ span ></ tt > is < tt class = "docutils literal" >< span class = "pre" > use_default</ span ></ tt > then the < tt class = "docutils literal" >< span class = "pre" > reference</ span ></ tt > member of
< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > is
< tt class = "docutils literal" >< span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type</ span ></ tt > .
Otherwise, < tt class = "docutils literal" >< span class = "pre" > reference</ span ></ tt > is < tt class = "docutils literal" >< span class = "pre" > Reference</ span ></ tt > .</ p >
< p > If < tt class = "docutils literal" >< span class = "pre" > Value</ span ></ tt > is < tt class = "docutils literal" >< span class = "pre" > use_default</ span ></ tt > then the < tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > member is
< tt class = "docutils literal" >< span class = "pre" > remove_cv< remove_reference< reference> </ span > < span class = "pre" > > ::type</ span ></ tt > . Otherwise,
< tt class = "docutils literal" >< span class = "pre" > value_type</ span ></ tt > is < tt class = "docutils literal" >< span class = "pre" > Value</ span ></ tt > .</ p >
< p > If < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > models Readable Lvalue Iterator and if < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt >
models Random Access Traversal Iterator, then < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > is
convertible to < tt class = "docutils literal" >< span class = "pre" > random_access_iterator_tag</ span ></ tt > . Otherwise, if
< tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > models Bidirectional Traversal Iterator, then
< tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > is convertible to
< tt class = "docutils literal" >< span class = "pre" > bidirectional_iterator_tag</ span ></ tt > . Otherwise < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > is
convertible to < tt class = "docutils literal" >< span class = "pre" > forward_iterator_tag</ span ></ tt > . If < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > does not
model Readable Lvalue Iterator then < tt class = "docutils literal" >< span class = "pre" > iterator_category</ span ></ tt > is
convertible to < tt class = "docutils literal" >< span class = "pre" > input_iterator_tag</ span ></ tt > .</ p >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id3" id = "transform-iterator-requirements" name = "transform-iterator-requirements" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > requirements</ a ></ h1 >
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< p > The type < tt class = "docutils literal" >< span class = "pre" > UnaryFunction</ span ></ tt > must be Assignable, Copy Constructible, and
the expression < tt class = "docutils literal" >< span class = "pre" > f(*i)</ span ></ tt > must be valid where < tt class = "docutils literal" >< span class = "pre" > f</ span ></ tt > is an object of
type < tt class = "docutils literal" >< span class = "pre" > UnaryFunction</ span ></ tt > , < tt class = "docutils literal" >< span class = "pre" > i</ span ></ tt > is an object of type < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > , and
where the type of < tt class = "docutils literal" >< span class = "pre" > f(*i)</ span ></ tt > must be
< tt class = "docutils literal" >< span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type</ span ></ tt > .</ p >
< p > The argument < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > shall model Readable Iterator.</ p >
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</ div >
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< div class = "section" >
< h1 >< a class = "toc-backref" href = "#id4" id = "transform-iterator-models" name = "transform-iterator-models" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models</ a ></ h1 >
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< p > The resulting < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models the most refined of the
following that is also modeled by < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > .</ p >
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< blockquote >
< ul class = "simple" >
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< li > Writable Lvalue Iterator if < tt class = "docutils literal" >< span class = "pre" > transform_iterator::reference</ span ></ tt > is a non-const reference.</ li >
< li > Readable Lvalue Iterator if < tt class = "docutils literal" >< span class = "pre" > transform_iterator::reference</ span ></ tt > is a const reference.</ li >
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< li > Readable Iterator otherwise.</ li >
</ ul >
</ blockquote >
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< p > The < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models the most refined standard traversal
concept that is modeled by the < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > argument.</ p >
< p > If < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > is a model of Readable Lvalue Iterator then
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it models the following original iterator concepts depending on what
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the < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > argument models.</ p >
< table border = "1" class = "docutils" >
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< colgroup >
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< col width = "47%" />
< col width = "53%" />
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</ colgroup >
< thead valign = "bottom" >
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< tr >< th class = "head" > If < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > models</ th >
< th class = "head" > then < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models</ th >
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</ tr >
</ thead >
< tbody valign = "top" >
< tr >< td > Single Pass Iterator</ td >
< td > Input Iterator</ td >
</ tr >
< tr >< td > Forward Traversal Iterator</ td >
< td > Forward Iterator</ td >
</ tr >
< tr >< td > Bidirectional Traversal Iterator</ td >
< td > Bidirectional Iterator</ td >
</ tr >
< tr >< td > Random Access Traversal Iterator</ td >
< td > Random Access Iterator</ td >
</ tr >
</ tbody >
</ table >
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< p > If < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > models Writable Lvalue Iterator then it is a
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mutable iterator (as defined in the old iterator requirements).</ p >
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< p >< tt class = "docutils literal" >< span class = "pre" > transform_iterator< F1,</ span > < span class = "pre" > X,</ span > < span class = "pre" > R1,</ span > < span class = "pre" > V1> </ span ></ tt > is interoperable with
< tt class = "docutils literal" >< span class = "pre" > transform_iterator< F2,</ span > < span class = "pre" > Y,</ span > < span class = "pre" > R2,</ span > < span class = "pre" > V2> </ 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 = "transform-iterator-operations" name = "transform-iterator-operations" >< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > operations</ a ></ h1 >
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< p > In addition to the operations required by the concepts modeled by
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< tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > , < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > provides the following
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operations.</ p >
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< p >< tt class = "docutils literal" >< span class = "pre" > transform_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" > Returns:</ th >< td class = "field-body" > An instance of < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > with < tt class = "docutils literal" >< span class = "pre" > m_f</ span ></ tt >
and < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > default constructed.</ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > transform_iterator(Iterator</ span > < span class = "pre" > const& </ span > < span class = "pre" > x,</ span > < span class = "pre" > UnaryFunction</ span > < span class = "pre" > f);</ 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" > An instance of < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > with < tt class = "docutils literal" >< span class = "pre" > m_f</ span ></ tt >
initialized to < tt class = "docutils literal" >< span class = "pre" > f</ span ></ tt > and < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > initialized to < tt class = "docutils literal" >< span class = "pre" > x</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
< pre class = "literal-block" >
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template< class F2, class I2, class R2, class V2>
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transform_iterator(
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transform_iterator< F2, I2, R2, V2> const& t
, typename enable_if_convertible< I2, Iterator> ::type* = 0 // exposition only
, typename enable_if_convertible< F2, UnaryFunction> ::type* = 0 // exposition only
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);
</ 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" > Returns:</ th >< td class = "field-body" > An instance of < tt class = "docutils literal" >< span class = "pre" > transform_iterator</ span ></ tt > with < tt class = "docutils literal" >< span class = "pre" > m_f</ span ></ tt >
initialized to < tt class = "docutils literal" >< span class = "pre" > t.functor()</ span ></ tt > and < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > initialized to
< tt class = "docutils literal" >< span class = "pre" > t.base()</ span ></ tt > .</ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Requires:</ th >< td class = "field-body" >< tt class = "docutils literal" >< span class = "pre" > OtherIterator</ span ></ tt > is implicitly convertible to < tt class = "docutils literal" >< span class = "pre" > Iterator</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > UnaryFunction</ span > < span class = "pre" > functor()</ 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_f</ 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_f(*m_iterator)</ span ></ tt ></ td >
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</ tr >
</ tbody >
</ table >
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< p >< tt class = "docutils literal" >< span class = "pre" > transform_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" > transform_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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< pre class = "literal-block" >
template < class UnaryFunction, class Iterator>
transform_iterator< UnaryFunction, Iterator>
make_transform_iterator(Iterator it, UnaryFunction fun);
</ 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" > Returns:</ th >< td class = "field-body" > An instance of < tt class = "docutils literal" >< span class = "pre" > transform_iterator< UnaryFunction,</ span > < span class = "pre" > Iterator> </ span ></ tt > with < tt class = "docutils literal" >< span class = "pre" > m_f</ span ></ tt >
initialized to < tt class = "docutils literal" >< span class = "pre" > f</ span ></ tt > and < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > initialized to < tt class = "docutils literal" >< span class = "pre" > x</ span ></ tt > .</ td >
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</ tr >
</ tbody >
</ table >
< pre class = "literal-block" >
template < class UnaryFunction, class Iterator>
transform_iterator< UnaryFunction, Iterator>
make_transform_iterator(Iterator it);
</ 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" > Returns:</ th >< td class = "field-body" > An instance of < tt class = "docutils literal" >< span class = "pre" > transform_iterator< UnaryFunction,</ span > < span class = "pre" > Iterator> </ span ></ tt > with < tt class = "docutils literal" >< span class = "pre" > m_f</ span ></ tt >
default constructed and < tt class = "docutils literal" >< span class = "pre" > m_iterator</ span ></ tt > initialized to < tt class = "docutils literal" >< span class = "pre" > x</ 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 is a simple example of using the transform_iterators class to
generate iterators that multiply (or add to) the value returned by
dereferencing the iterator. It would be cooler to use lambda library
in this example.</ p >
< pre class = "literal-block" >
int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
const int N = sizeof(x)/sizeof(int);
typedef boost::binder1st< std::multiplies< int> > Function;
typedef boost::transform_iterator< Function, int*> doubling_iterator;
doubling_iterator i(x, boost::bind1st(std::multiplies< int> (), 2)),
i_end(x + N, boost::bind1st(std::multiplies< int> (), 2));
std::cout << " multiplying the array by 2:" << std::endl;
while (i != i_end)
std::cout << *i++ << " " ;
std::cout << std::endl;
std::cout << " adding 4 to each element in the array:" << std::endl;
std::copy(boost::make_transform_iterator(x, boost::bind1st(std::plus< int> (), 4)),
boost::make_transform_iterator(x + N, boost::bind1st(std::plus< int> (), 4)),
std::ostream_iterator< int> (std::cout, " " ));
std::cout << std::endl;
</ pre >
< p > The output is:</ p >
< pre class = "literal-block" >
multiplying the array by 2:
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2 4 6 8 10 12 14 16
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adding 4 to each element in the array:
5 6 7 8 9 10 11 12
</ pre >
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< p > The source code for this example can be found < a class = "reference" href = "../example/transform_iterator_example.cpp" > here</ a > .</ p >
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</ div >
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</ div >
</ body >
</ html >
