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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" /> |
| <meta name="date" content="2006-09-11" /> |
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| <div class="document" id="counting-iterator"> |
| <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 external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" 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 external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems |
| Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport |
| Railway Operation and Construction</a></td></tr> |
| <tr><th class="docinfo-name">Date:</th> |
| <td>2006-09-11</td></tr> |
| <tr><th class="docinfo-name">Copyright:</th> |
| <td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr> |
| </tbody> |
| </table> |
| <!-- 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) --> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <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 |
| through one hundred using <tt class="docutils literal"><span class="pre">std::copy()</span></tt>? The only iterator |
| operation missing from builtin integer types is an |
| <tt class="docutils literal"><span class="pre">operator*()</span></tt> that returns the current value of the integer. |
| 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 |
| any incrementable type.</p> |
| <!-- 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) --> |
| <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 |
| returns the current value of the object. All other iterator operations |
| are forwarded to the adapted object.</p> |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="contents topic" id="table-of-contents"> |
| <p class="topic-title first">Table of Contents</p> |
| <ul class="simple"> |
| <li><a class="reference internal" href="#counting-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> synopsis</a></li> |
| <li><a class="reference internal" href="#counting-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> requirements</a></li> |
| <li><a class="reference internal" href="#counting-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models</a></li> |
| <li><a class="reference internal" href="#counting-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> operations</a></li> |
| <li><a class="reference internal" href="#example" id="id6">Example</a></li> |
| </ul> |
| </div> |
| <div class="section" id="counting-iterator-synopsis"> |
| <h1><a class="toc-backref" href="#id2"><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) --> |
| <pre class="literal-block"> |
| template < |
| class Incrementable |
| , class CategoryOrTraversal = use_default |
| , class Difference = use_default |
| > |
| class counting_iterator |
| { |
| public: |
| typedef Incrementable value_type; |
| typedef const Incrementable& reference; |
| typedef const Incrementable* pointer; |
| typedef /* see below */ difference_type; |
| typedef /* see below */ iterator_category; |
| |
| counting_iterator(); |
| counting_iterator(counting_iterator const& rhs); |
| explicit counting_iterator(Incrementable x); |
| Incrementable const& base() const; |
| reference operator*() const; |
| counting_iterator& operator++(); |
| counting_iterator& operator--(); |
| private: |
| Incrementable m_inc; // exposition |
| }; |
| </pre> |
| <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 |
| 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 external" href="iterator_facade.html#iterator-category"><em>iterator-category</em></a>( |
| random_access_traversal_tag, Incrementable, const Incrementable&) |
| else |
| return <a class="reference external" href="iterator_facade.html#iterator-category"><em>iterator-category</em></a>( |
| iterator_traversal<Incrementable>::type, |
| Incrementable, const Incrementable&) |
| </pre> |
| <dl class="docutils"> |
| <dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt> |
| <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> |
| is true.]</dd> |
| </dl> |
| </div> |
| <div class="section" id="counting-iterator-requirements"> |
| <h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> requirements</a></h1> |
| <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> |
| <pre class="literal-block"> |
| Incrementable i, j; |
| ++i; // pre-increment |
| i == j; // operator equal |
| </pre> |
| <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>, |
| the following expression must also be well-formed:</p> |
| <pre class="literal-block"> |
| --i |
| </pre> |
| <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>, |
| the following must must also be valid:</p> |
| <pre class="literal-block"> |
| counting_iterator::difference_type n; |
| i += n; |
| n = i - j; |
| i < j; |
| </pre> |
| </div> |
| <div class="section" id="counting-iterator-models"> |
| <h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> models</a></h1> |
| <p>Specializations of <tt class="docutils literal"><span class="pre">counting_iterator</span></tt> model Readable Lvalue |
| Iterator. In addition, they model the concepts corresponding to the |
| 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> |
| </div> |
| <div class="section" id="counting-iterator-operations"> |
| <h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt> operations</a></h1> |
| <p>In addition to the operations required by the concepts modeled by |
| <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 |
| operations.</p> |
| <p><tt class="docutils literal"><span class="pre">counting_iterator();</span></tt></p> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <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> |
| </tr> |
| <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> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <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> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <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> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <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="docutils literal"><span class="pre">m_inc</span></tt></td> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <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="docutils literal"><span class="pre">++m_inc</span></tt></td> |
| </tr> |
| <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <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="docutils literal"><span class="pre">--m_inc</span></tt></td> |
| </tr> |
| <tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <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="docutils literal"><span class="pre">m_inc</span></tt></td> |
| </tr> |
| </tbody> |
| </table> |
| <!-- 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) --> |
| <pre class="literal-block"> |
| template <class Incrementable> |
| counting_iterator<Incrementable> make_counting_iterator(Incrementable x); |
| </pre> |
| <table class="docutils 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="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> |
| </tr> |
| </tbody> |
| </table> |
| <!-- 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) --> |
| </div> |
| <div class="section" id="example"> |
| <h1><a class="toc-backref" href="#id6">Example</a></h1> |
| <p>This example fills an array with numbers and a second array with |
| 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 |
| 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)); |
| |
| std::cout << "indirectly printing out the numbers from 0 to " |
| << 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 |
| 0 1 2 3 4 5 6 |
| </pre> |
| <p>The source code for this example can be found <a class="reference external" href="../example/counting_iterator_example.cpp">here</a>.</p> |
| </div> |
| </div> |
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