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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" />
- <meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
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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>
- <div class="footer">
- <hr class="footer" />
- <a class="reference external" href="indirect_iterator.rst">View document source</a>.
- Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
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