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| <title>Indirect Iterator</title> |
| <meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" /> |
| <meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" /> |
| <meta name="date" content="2006-09-11" /> |
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| <div class="document" id="indirect-iterator"> |
| <h1 class="title">Indirect Iterator</h1> |
| <table class="docinfo" frame="void" rules="none"> |
| <col class="docinfo-name" /> |
| <col class="docinfo-content" /> |
| <tbody valign="top"> |
| <tr><th class="docinfo-name">Author:</th> |
| <td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr> |
| <tr><th class="docinfo-name">Contact:</th> |
| <td><a class="first reference 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"><!-- Copyright David Abrahams 2006. Distributed under the Boost --> |
| <!-- Software License, Version 1.0. (See accompanying --> |
| <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> |
| <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> adapts an iterator by applying an |
| <em>extra</em> dereference inside of <tt class="docutils literal"><span class="pre">operator*()</span></tt>. For example, this |
| iterator adaptor makes it possible to view a container of pointers |
| (e.g. <tt class="docutils literal"><span class="pre">list<foo*></span></tt>) as if it were a container of the pointed-to type |
| (e.g. <tt class="docutils literal"><span class="pre">list<foo></span></tt>). <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> depends on two |
| auxiliary traits, <tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>, to |
| provide support for underlying iterators whose <tt class="docutils literal"><span class="pre">value_type</span></tt> is |
| not an iterator.</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="#indirect-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></li> |
| <li><a class="reference internal" href="#indirect-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li> |
| <li><a class="reference internal" href="#indirect-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></li> |
| <li><a class="reference internal" href="#indirect-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></li> |
| <li><a class="reference internal" href="#example" id="id6">Example</a></li> |
| </ul> |
| </div> |
| <div class="section" id="indirect-iterator-synopsis"> |
| <h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></h1> |
| <!-- Copyright David Abrahams 2006. Distributed under the Boost --> |
| <!-- Software License, Version 1.0. (See accompanying --> |
| <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> |
| <pre class="literal-block"> |
| template < |
| class Iterator |
| , class Value = use_default |
| , class CategoryOrTraversal = use_default |
| , class Reference = use_default |
| , class Difference = use_default |
| > |
| class indirect_iterator |
| { |
| public: |
| typedef /* see below */ value_type; |
| typedef /* see below */ reference; |
| typedef /* see below */ pointer; |
| typedef /* see below */ difference_type; |
| typedef /* see below */ iterator_category; |
| |
| indirect_iterator(); |
| indirect_iterator(Iterator x); |
| |
| template < |
| class Iterator2, class Value2, class Category2 |
| , class Reference2, class Difference2 |
| > |
| indirect_iterator( |
| indirect_iterator< |
| Iterator2, Value2, Category2, Reference2, Difference2 |
| > const& y |
| , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition |
| ); |
| |
| Iterator const& base() const; |
| reference operator*() const; |
| indirect_iterator& operator++(); |
| indirect_iterator& operator--(); |
| private: |
| Iterator m_iterator; // exposition |
| }; |
| </pre> |
| <p>The member types of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> are defined according to |
| the following pseudo-code, where <tt class="docutils literal"><span class="pre">V</span></tt> is |
| <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt></p> |
| <pre class="literal-block"> |
| if (Value is use_default) then |
| typedef remove_const<pointee<V>::type>::type value_type; |
| else |
| typedef remove_const<Value>::type value_type; |
| |
| if (Reference is use_default) then |
| if (Value is use_default) then |
| typedef indirect_reference<V>::type reference; |
| else |
| typedef Value& reference; |
| else |
| typedef Reference reference; |
| |
| if (Value is use_default) then |
| typedef pointee<V>::type* pointer; |
| else |
| typedef Value* pointer; |
| |
| if (Difference is use_default) |
| typedef iterator_traits<Iterator>::difference_type difference_type; |
| else |
| typedef Difference difference_type; |
| |
| if (CategoryOrTraversal is use_default) |
| typedef <em>iterator-category</em> ( |
| iterator_traversal<Iterator>::type,``reference``,``value_type`` |
| ) iterator_category; |
| else |
| typedef <em>iterator-category</em> ( |
| CategoryOrTraversal,``reference``,``value_type`` |
| ) iterator_category; |
| </pre> |
| </div> |
| <div class="section" id="indirect-iterator-requirements"> |
| <h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h1> |
| <p>The expression <tt class="docutils literal"><span class="pre">*v</span></tt>, where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of |
| <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>, shall be valid |
| expression and convertible to <tt class="docutils literal"><span class="pre">reference</span></tt>. <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall |
| model the traversal concept indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>. |
| <tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt> shall be chosen so |
| that <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt> meet |
| the requirements indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p> |
| <p>[Note: there are further requirements on the |
| <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt> if the <tt class="docutils literal"><span class="pre">Value</span></tt> |
| parameter is not <tt class="docutils literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for |
| deducing the default for the <tt class="docutils literal"><span class="pre">value_type</span></tt> member.]</p> |
| </div> |
| <div class="section" id="indirect-iterator-models"> |
| <h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models</a></h1> |
| <p>In addition to the concepts indicated by <tt class="docutils literal"><span class="pre">iterator_category</span></tt> |
| and by <tt class="docutils literal"><span class="pre">iterator_traversal<indirect_iterator>::type</span></tt>, a |
| specialization of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> models the following |
| concepts, Where <tt class="docutils literal"><span class="pre">v</span></tt> is an object of |
| <tt class="docutils literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>:</p> |
| <blockquote> |
| <ul class="simple"> |
| <li>Readable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span></tt> is convertible to |
| <tt class="docutils literal"><span class="pre">value_type</span></tt>.</li> |
| <li>Writable Iterator if <tt class="docutils literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid |
| expression (where <tt class="docutils literal"><span class="pre">t</span></tt> is an object of type |
| <tt class="docutils literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li> |
| <li>Lvalue Iterator if <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type.</li> |
| </ul> |
| </blockquote> |
| <p><tt class="docutils literal"><span class="pre">indirect_iterator<X,V1,C1,R1,D1></span></tt> is interoperable with |
| <tt class="docutils literal"><span class="pre">indirect_iterator<Y,V2,C2,R2,D2></span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is |
| interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p> |
| </div> |
| <div class="section" id="indirect-iterator-operations"> |
| <h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> operations</a></h1> |
| <p>In addition to the operations required by the concepts described |
| above, specializations of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> provide the |
| following operations.</p> |
| <p><tt class="docutils literal"><span class="pre">indirect_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">Iterator</span></tt> must be Default Constructible.</td> |
| </tr> |
| <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with |
| a default-constructed <tt class="docutils literal"><span class="pre">m_iterator</span></tt>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p><tt class="docutils literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> with |
| <tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <pre class="literal-block"> |
| template < |
| class Iterator2, class Value2, unsigned Access, class Traversal |
| , class Reference2, class Difference2 |
| > |
| indirect_iterator( |
| indirect_iterator< |
| Iterator2, Value2, Access, Traversal, Reference2, Difference2 |
| > const& y |
| , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition |
| ); |
| </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">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td> |
| </tr> |
| <tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> whose |
| <tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <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"> |
| <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_iterator</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_iterator</span></tt></td> |
| </tr> |
| </tbody> |
| </table> |
| <p><tt class="docutils literal"><span class="pre">indirect_iterator&</span> <span class="pre">operator++();</span></tt></p> |
| <table class="docutils field-list" frame="void" rules="none"> |
| <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_iterator</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">indirect_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_iterator</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> |
| <!-- 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 prints an array of characters, using |
| <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> to access the array of characters through an |
| array of pointers. Next <tt class="docutils literal"><span class="pre">indirect_iterator</span></tt> is used with the |
| <tt class="docutils literal"><span class="pre">transform</span></tt> algorithm to copy the characters (incremented by one) to |
| another array. A constant indirect iterator is used for the source and |
| a mutable indirect iterator is used for the destination. The last part |
| of the example prints the original array of characters, but this time |
| using the <tt class="docutils literal"><span class="pre">make_indirect_iterator</span></tt> helper function.</p> |
| <pre class="literal-block"> |
| char characters[] = "abcdefg"; |
| const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char |
| char* pointers_to_chars[N]; // at the end. |
| for (int i = 0; i < N; ++i) |
| pointers_to_chars[i] = &characters[i]; |
| |
| // Example of using indirect_iterator |
| |
| boost::indirect_iterator<char**, char> |
| indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N); |
| |
| std::copy(indirect_first, indirect_last, std::ostream_iterator<char>(std::cout, ",")); |
| std::cout << std::endl; |
| |
| |
| // Example of making mutable and constant indirect iterators |
| |
| char mutable_characters[N]; |
| char* pointers_to_mutable_chars[N]; |
| for (int j = 0; j < N; ++j) |
| pointers_to_mutable_chars[j] = &mutable_characters[j]; |
| |
| boost::indirect_iterator<char* const*> mutable_indirect_first(pointers_to_mutable_chars), |
| mutable_indirect_last(pointers_to_mutable_chars + N); |
| boost::indirect_iterator<char* const*, char const> const_indirect_first(pointers_to_chars), |
| const_indirect_last(pointers_to_chars + N); |
| |
| std::transform(const_indirect_first, const_indirect_last, |
| mutable_indirect_first, std::bind1st(std::plus<char>(), 1)); |
| |
| std::copy(mutable_indirect_first, mutable_indirect_last, |
| std::ostream_iterator<char>(std::cout, ",")); |
| std::cout << std::endl; |
| |
| |
| // Example of using make_indirect_iterator() |
| |
| std::copy(boost::make_indirect_iterator(pointers_to_chars), |
| boost::make_indirect_iterator(pointers_to_chars + N), |
| std::ostream_iterator<char>(std::cout, ",")); |
| std::cout << std::endl; |
| </pre> |
| <p>The output is:</p> |
| <pre class="literal-block"> |
| a,b,c,d,e,f,g, |
| b,c,d,e,f,g,h, |
| a,b,c,d,e,f,g, |
| </pre> |
| <p>The source code for this example can be found <a class="reference external" href="../example/indirect_iterator_example.cpp">here</a>.</p> |
| </div> |
| </div> |
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