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< title > Counting 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" / >
2004-11-02 14:31:27 +00:00
< meta name = "date" content = "2004-11-01" / >
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< / style >
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< / head >
< body >
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< div class = "document" id = "counting-iterator" >
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< h1 class = "title" > Counting 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" > < p class = "first" > How would you fill up a vector with the numbers zero
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through one hundred using < tt class = "docutils literal" > < span class = "pre" > std::copy()< / span > < / tt > ? The only iterator
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operation missing from builtin integer types is an
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< tt class = "docutils literal" > < span class = "pre" > operator*()< / span > < / tt > that returns the current value of the integer.
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The counting iterator adaptor adds this crucial piece of
functionality to whatever type it wraps. One can use the
counting iterator adaptor not only with integer types, but with
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any incrementable type.< / p >
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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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< p class = "last" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > adapts an object by adding an < tt class = "docutils literal" > < span class = "pre" > operator*< / span > < / tt > that
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returns the current value of the object. All other iterator operations
are forwarded to the adapted object.< / p >
< / 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 = "#counting-iterator-synopsis" id = "id2" name = "id2" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > synopsis< / a > < / li >
< li > < a class = "reference" href = "#counting-iterator-requirements" id = "id3" name = "id3" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > requirements< / a > < / li >
< li > < a class = "reference" href = "#counting-iterator-models" id = "id4" name = "id4" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > models< / a > < / li >
< li > < a class = "reference" href = "#counting-iterator-operations" id = "id5" name = "id5" > < tt class = "docutils literal" > < span class = "pre" > counting_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 = "counting-iterator-synopsis" name = "counting-iterator-synopsis" > < tt class = "docutils literal" > < span class = "pre" > counting_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" >
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template <
class Incrementable
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, class CategoryOrTraversal = use_default
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, class Difference = use_default
>
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class counting_iterator
{
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public:
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typedef Incrementable value_type;
typedef const Incrementable& reference;
typedef const Incrementable* pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
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counting_iterator();
counting_iterator(counting_iterator const& rhs);
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explicit counting_iterator(Incrementable x);
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Incrementable const& base() const;
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reference operator*() const;
counting_iterator& operator++();
counting_iterator& operator--();
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private:
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Incrementable m_inc; // exposition
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};
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< / pre >
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< p > If the < tt class = "docutils literal" > < span class = "pre" > Difference< / span > < / tt > argument is < tt class = "docutils literal" > < span class = "pre" > use_default< / span > < / tt > then
< tt class = "docutils literal" > < span class = "pre" > difference_type< / span > < / tt > is an unspecified signed integral
type. Otherwise < tt class = "docutils literal" > < span class = "pre" > difference_type< / span > < / tt > is < tt class = "docutils literal" > < span class = "pre" > Difference< / span > < / tt > .< / p >
< p > < tt class = "docutils literal" > < span class = "pre" > iterator_category< / span > < / tt > is determined according to the following
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algorithm:< / p >
< pre class = "literal-block" >
if (CategoryOrTraversal is not use_default)
return CategoryOrTraversal
else if (numeric_limits< Incrementable> ::is_specialized)
return < a class = "reference" href = "iterator_facade.html#iterator-category" > < em > iterator-category< / em > < / a > (
random_access_traversal_tag, Incrementable, const Incrementable& )
else
return < a class = "reference" href = "iterator_facade.html#iterator-category" > < em > iterator-category< / em > < / a > (
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iterator_traversal< Incrementable> ::type,
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Incrementable, const Incrementable& )
< / pre >
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< dl class = "docutils" >
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< dt > [< em > Note:< / em > implementers are encouraged to provide an implementation of< / dt >
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< dd > < tt class = "docutils literal" > < span class = "pre" > operator-< / span > < / tt > and a < tt class = "docutils literal" > < span class = "pre" > difference_type< / span > < / tt > that avoids overflows in
the cases where < tt class = "docutils literal" > < span class = "pre" > std::numeric_limits< Incrementable> ::is_specialized< / span > < / tt >
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is true.]< / dd >
< / dl >
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< / div >
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< div class = "section" >
< h1 > < a class = "toc-backref" href = "#id3" id = "counting-iterator-requirements" name = "counting-iterator-requirements" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > requirements< / a > < / h1 >
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< p > The < tt class = "docutils literal" > < span class = "pre" > Incrementable< / span > < / tt > argument shall be Copy Constructible and Assignable.< / p >
< p > If < 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 >
or < tt class = "docutils literal" > < span class = "pre" > forward_traversal_tag< / span > < / tt > , the following must be well-formed:< / p >
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< pre class = "literal-block" >
Incrementable i, j;
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++i; // pre-increment
i == j; // operator equal
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< / pre >
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< p > If < 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 > or < tt class = "docutils literal" > < span class = "pre" > bidirectional_traversal_tag< / span > < / tt > ,
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the following expression must also be well-formed:< / p >
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< pre class = "literal-block" >
--i
< / pre >
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< p > If < 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 > or < tt class = "docutils literal" > < span class = "pre" > random_access_traversal_tag< / span > < / tt > ,
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the following must must also be valid:< / p >
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< pre class = "literal-block" >
counting_iterator::difference_type n;
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i += n;
n = i - j;
i < j;
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< / pre >
< / div >
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< div class = "section" >
< h1 > < a class = "toc-backref" href = "#id4" id = "counting-iterator-models" name = "counting-iterator-models" > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > models< / a > < / h1 >
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< p > Specializations of < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > model Readable Lvalue
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Iterator. In addition, they model the concepts corresponding to the
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iterator tags to which their < tt class = "docutils literal" > < span class = "pre" > iterator_category< / span > < / tt > is convertible.
Also, if < tt class = "docutils literal" > < span class = "pre" > CategoryOrTraversal< / span > < / tt > is not < tt class = "docutils literal" > < span class = "pre" > use_default< / span > < / tt > then
< tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > models the concept corresponding to the iterator
tag < tt class = "docutils literal" > < span class = "pre" > CategoryOrTraversal< / span > < / tt > . Otherwise, if
< tt class = "docutils literal" > < span class = "pre" > numeric_limits< Incrementable> ::is_specialized< / span > < / tt > , then
< tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > models Random Access Traversal Iterator.
Otherwise, < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > models the same iterator traversal
concepts modeled by < tt class = "docutils literal" > < span class = "pre" > Incrementable< / span > < / tt > .< / p >
< p > < tt class = "docutils literal" > < span class = "pre" > counting_iterator< X,C1,D1> < / span > < / tt > is interoperable with
< tt class = "docutils literal" > < span class = "pre" > counting_iterator< Y,C2,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 = "counting-iterator-operations" name = "counting-iterator-operations" > < tt class = "docutils literal" > < span class = "pre" > counting_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" > counting_iterator< / span > < / tt > , < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > provides the following
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operations.< / p >
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< p > < tt class = "docutils literal" > < span class = "pre" > counting_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" > Incrementable< / span > < / tt > is 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" > Default construct the member < tt class = "docutils literal" > < span class = "pre" > m_inc< / span > < / tt > .< / td >
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< / tr >
< / tbody >
< / table >
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< p > < tt class = "docutils literal" > < span class = "pre" > counting_iterator(counting_iterator< / span > < span class = "pre" > const& < / span > < span class = "pre" > rhs);< / 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" > Construct member < tt class = "docutils literal" > < span class = "pre" > m_inc< / span > < / tt > from < tt class = "docutils literal" > < span class = "pre" > rhs.m_inc< / span > < / tt > .< / td >
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< / tr >
< / tbody >
< / table >
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< p > < tt class = "docutils literal" > < span class = "pre" > explicit< / span > < span class = "pre" > counting_iterator(Incrementable< / 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" > Construct member < tt class = "docutils literal" > < span class = "pre" > m_inc< / span > < / tt > from < tt class = "docutils literal" > < span class = "pre" > x< / 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_inc< / span > < / tt > < / td >
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< / tr >
< / tbody >
< / table >
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< p > < tt class = "docutils literal" > < span class = "pre" > counting_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_inc< / 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" > counting_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_inc< / 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" > Incrementable< / 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_inc< / 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 Incrementable>
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counting_iterator< Incrementable> make_counting_iterator(Incrementable x);
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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" > counting_iterator< Incrementable> < / span > < / tt >
with < tt class = "docutils literal" > < span class = "pre" > current< / span > < / tt > constructed from < 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 example fills an array with numbers and a second array with
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pointers into the first array, using < tt class = "docutils literal" > < span class = "pre" > counting_iterator< / span > < / tt > for both
tasks. Finally < tt class = "docutils literal" > < span class = "pre" > indirect_iterator< / span > < / tt > is used to print out the numbers
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into the first array via indirection through the second array.< / p >
< pre class = "literal-block" >
int N = 7;
std::vector< int> numbers;
typedef std::vector< int> ::iterator n_iter;
std::copy(boost::counting_iterator< int> (0),
boost::counting_iterator< int> (N),
std::back_inserter(numbers));
std::vector< std::vector< int> ::iterator> pointers;
std::copy(boost::make_counting_iterator(numbers.begin()),
boost::make_counting_iterator(numbers.end()),
std::back_inserter(pointers));
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std::cout < < " indirectly printing out the numbers from 0 to "
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< < N < < std::endl;
std::copy(boost::make_indirect_iterator(pointers.begin()),
boost::make_indirect_iterator(pointers.end()),
std::ostream_iterator< int> (std::cout, " " ));
std::cout < < std::endl;
< / pre >
< p > The output is:< / p >
< pre class = "literal-block" >
indirectly printing out the numbers from 0 to 7
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0 1 2 3 4 5 6
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< / pre >
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< p > The source code for this example can be found < a class = "reference" href = "../example/counting_iterator_example.cpp" > here< / a > .< / p >
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< / div >
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< / div >
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< / body >
< / html >
