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< title > Transform 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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< meta name = "date" content = "2004-01-12" / >
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< meta name = "copyright" content = "Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved" / >
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< div class = "document" id = "transform-iterator" >
< h1 class = "title" > Transform Iterator< / h1 >
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< 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 > 2004-01-12< / 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. All rights reserved< / td > < / tr >
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< tr class = "field" > < th class = "field-name" > abstract:< / th > < td class = "field-body" > < / td >
< / tr >
< / tbody >
< / table >
< p > The transform iterator adapts an iterator by applying some function
object to the result of dereferencing the iterator. In other words,
the < tt class = "literal" > < span class = "pre" > operator*< / span > < / tt > of the transform iterator first dereferences the
base iterator, passes the result of this to the function object, and
then returns the result.< / p >
< div class = "contents topic" id = "table-of-contents" >
< p class = "topic-title" > < a name = "table-of-contents" > Table of Contents< / a > < / p >
< ul class = "simple" >
< li > < a class = "reference" href = "#transform-iterator-requirements" id = "id1" name = "id1" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > requirements< / a > < / li >
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< li > < a class = "reference" href = "#transform-iterator-models" id = "id2" name = "id2" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > models< / a > < / li >
< li > < a class = "reference" href = "#transform-iterator-operations" id = "id3" name = "id3" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > operations< / a > < / li >
< li > < a class = "reference" href = "#example" id = "id4" name = "id4" > Example< / a > < / li >
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< / ul >
< / div >
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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,
class Value = use_default>
class transform_iterator
{
public:
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
, typename enable_if_convertible< I2, Iterator> ::type* = 0 // exposition
, typename enable_if_convertible< F2, UnaryFunction> ::type* = 0 // exposition
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);
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reference operator*() const;
transform_iterator& operator++();
Iterator base() const;
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UnaryFunction functor() const;
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private:
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Iterator m_iterator; // exposition
UnaryFunction m_f; // exposition
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};
< / pre >
< div class = "section" id = "transform-iterator-requirements" >
< h1 > < a class = "toc-backref" href = "#id1" name = "transform-iterator-requirements" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > requirements< / a > < / h1 >
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< p > The type < tt class = "literal" > < span class = "pre" > UnaryFunction< / span > < / tt > must be Assignable, Copy Constructible, and
the expression < tt class = "literal" > < span class = "pre" > f(*i)< / span > < / tt > must be valid where < tt class = "literal" > < span class = "pre" > f< / span > < / tt > is an object of
type < tt class = "literal" > < span class = "pre" > UnaryFunction< / span > < / tt > , < tt class = "literal" > < span class = "pre" > i< / span > < / tt > is an object of type < tt class = "literal" > < span class = "pre" > Iterator< / span > < / tt > , and
where the type of < tt class = "literal" > < span class = "pre" > f(*i)< / span > < / tt > must be
< tt class = "literal" > < span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type< / span > < / tt > .< / p >
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< p > The type < tt class = "literal" > < span class = "pre" > Iterator< / span > < / tt > must at least model Readable Iterator.< / p >
< / div >
< div class = "section" id = "transform-iterator-models" >
< h1 > < a class = "toc-backref" href = "#id2" name = "transform-iterator-models" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > models< / a > < / h1 >
< p > The resulting < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > models the most refined of the
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following options that is also modeled by < tt class = "literal" > < span class = "pre" > Iterator< / span > < / tt > .< / p >
< blockquote >
< ul class = "simple" >
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< li > Writable Lvalue Iterator if < tt class = "literal" > < span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type< / span > < / tt > is a non-const reference.< / li >
< li > Readable Lvalue Iterator if < tt class = "literal" > < span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type< / span > < / tt > is a const
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reference.< / li >
< li > Readable Iterator otherwise.< / li >
< / ul >
< / blockquote >
< p > The < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > models the most refined standard traversal
concept that is modeled by < tt class = "literal" > < span class = "pre" > Iterator< / span > < / tt > .< / p >
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< p > The < tt class = "literal" > < span class = "pre" > reference< / span > < / tt > type of < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > is
< tt class = "literal" > < span class = "pre" > result_of< UnaryFunction(iterator_traits< Iterator> ::reference)> ::type< / span > < / tt > .
The < tt class = "literal" > < span class = "pre" > value_type< / span > < / tt > is < tt class = "literal" > < span class = "pre" > remove_cv< remove_reference< reference> < / span > < span class = "pre" > > ::type< / span > < / tt > .< / p >
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< / div >
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< div class = "section" id = "transform-iterator-operations" >
< h1 > < a class = "toc-backref" href = "#id3" name = "transform-iterator-operations" > < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > operations< / a > < / h1 >
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< p > < tt class = "literal" > < span class = "pre" > transform_iterator();< / span > < / tt > < / p >
< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > An instance of < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > with < tt class = "literal" > < span class = "pre" > m_f< / span > < / tt >
and < tt class = "literal" > < span class = "pre" > m_iterator< / span > < / tt > default constructed.< / td >
< / tr >
< / tbody >
< / table >
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< p > < tt class = "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 >
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< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > An instance of < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > with < tt class = "literal" > < span class = "pre" > m_f< / span > < / tt >
initialized to < tt class = "literal" > < span class = "pre" > f< / span > < / tt > and < tt class = "literal" > < span class = "pre" > m_iterator< / span > < / tt > initialized to < tt class = "literal" > < span class = "pre" > x< / span > < / tt > .< / td >
< / tr >
< / tbody >
< / table >
< pre class = "literal-block" >
template< class OtherIterator, class R2, class V2>
transform_iterator(
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transform_iterator< UnaryFunction, OtherIterator, R2, V2> const& t
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, typename enable_if_convertible< OtherIterator, Iterator> ::type* = 0 // exposition
);
< / pre >
< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > An instance of < tt class = "literal" > < span class = "pre" > transform_iterator< / span > < / tt > that is a copy of < tt class = "literal" > < span class = "pre" > t< / span > < / tt > .< / td >
< / tr >
< tr class = "field" > < th class = "field-name" > Requires:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > OtherIterator< / span > < / tt > is implicitly convertible to < tt class = "literal" > < span class = "pre" > Iterator< / span > < / tt > .< / td >
< / tr >
< / tbody >
< / table >
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< p > < tt class = "literal" > < span class = "pre" > Iterator< / span > < span class = "pre" > base()< / span > < span class = "pre" > const;< / span > < / tt > < / p >
< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > m_iterator< / span > < / tt > < / td >
< / tr >
< / tbody >
< / table >
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< p > < tt class = "literal" > < span class = "pre" > UnaryFunction< / span > < span class = "pre" > functor()< / span > < span class = "pre" > const;< / span > < / tt > < / p >
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< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > m_f< / span > < / tt > < / td >
< / tr >
< / tbody >
< / table >
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< p > < tt class = "literal" > < span class = "pre" > reference< / span > < span class = "pre" > operator*()< / span > < span class = "pre" > const;< / span > < / tt > < / p >
< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > m_f(*m_iterator)< / span > < / tt > < / td >
< / tr >
< / tbody >
< / table >
< p > < tt class = "literal" > < span class = "pre" > transform_iterator& < / span > < span class = "pre" > operator++();< / span > < / tt > < / p >
< table class = "field-list" frame = "void" rules = "none" >
< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
< tr class = "field" > < th class = "field-name" > Effects:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > ++m_iterator< / span > < / tt > < / td >
< / tr >
< tr class = "field" > < th class = "field-name" > Returns:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > *this< / span > < / tt > < / td >
< / tr >
< / tbody >
< / table >
< 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 = "field-list" frame = "void" rules = "none" >
< 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 = "literal" > < span class = "pre" > transform_iterator< UnaryFunction,< / span > < span class = "pre" > Iterator> < / span > < / tt > with < tt class = "literal" > < span class = "pre" > m_f< / span > < / tt >
initialized to < tt class = "literal" > < span class = "pre" > f< / span > < / tt > and < tt class = "literal" > < span class = "pre" > m_iterator< / span > < / tt > initialized to < tt class = "literal" > < span class = "pre" > x< / span > < / tt > .< / td >
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< / tr >
< / tbody >
< / table >
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< pre class = "literal-block" >
template < class UnaryFunction, class Iterator>
transform_iterator< UnaryFunction, Iterator>
make_transform_iterator(Iterator it);
< / pre >
< table class = "field-list" frame = "void" rules = "none" >
< 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 = "literal" > < span class = "pre" > transform_iterator< UnaryFunction,< / span > < span class = "pre" > Iterator> < / span > < / tt > with < tt class = "literal" > < span class = "pre" > m_f< / span > < / tt >
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default constructed and < tt class = "literal" > < span class = "pre" > m_iterator< / span > < / tt > initialized to < tt class = "literal" > < span class = "pre" > x< / span > < / tt > .< / td >
< / tr >
< / tbody >
< / table >
< / div >
< div class = "section" id = "example" >
< h1 > < a class = "toc-backref" href = "#id4" name = "example" > Example< / a > < / h1 >
< 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:
2 4 6 8 10 12 14 16
adding 4 to each element in the array:
5 6 7 8 9 10 11 12
< / pre >
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< / div >
< / div >
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< hr class = "footer" / >
< div class = "footer" >
< a class = "reference" href = "transform_iterator.rst" > View document source< / a > .
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