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7<title>Transform Iterator</title>
8<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
9<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
10<meta name="date" content="2006-09-11" />
11<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
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13</head>
14<body>
15<div class="document" id="transform-iterator">
16<h1 class="title">Transform Iterator</h1>
17<table class="docinfo" frame="void" rules="none">
18<col class="docinfo-name" />
19<col class="docinfo-content" />
20<tbody valign="top">
21<tr><th class="docinfo-name">Author:</th>
22<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
23<tr><th class="docinfo-name">Contact:</th>
24<td><a class="first reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference external" href="mailto:witt&#64;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</a></td></tr>
25<tr><th class="docinfo-name">Organization:</th>
26<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
27Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
28Railway Operation and Construction</a></td></tr>
29<tr><th class="docinfo-name">Date:</th>
30<td>2006-09-11</td></tr>
31<tr><th class="docinfo-name">Copyright:</th>
32<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
33</tbody>
34</table>
35<!-- Distributed under the Boost -->
36<!-- Software License, Version 1.0. (See accompanying -->
37<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
38<table class="docutils field-list" frame="void" rules="none">
39<col class="field-name" />
40<col class="field-body" />
41<tbody valign="top">
42<tr class="field"><th class="field-name">abstract:</th><td class="field-body"><!-- Copyright David Abrahams 2006. Distributed under the Boost -->
43<!-- Software License, Version 1.0. (See accompanying -->
44<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
45The transform iterator adapts an iterator by modifying the
46<tt class="docutils literal"><span class="pre">operator*</span></tt> to apply a function object to the result of
47dereferencing the iterator and returning the result.</td>
48</tr>
49</tbody>
50</table>
51<div class="contents topic" id="table-of-contents">
52<p class="topic-title first">Table of Contents</p>
53<ul class="simple">
54<li><a class="reference internal" href="#transform-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> synopsis</a></li>
55<li><a class="reference internal" href="#transform-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> requirements</a></li>
56<li><a class="reference internal" href="#transform-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</a></li>
57<li><a class="reference internal" href="#transform-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> operations</a></li>
58<li><a class="reference internal" href="#example" id="id6">Example</a></li>
59</ul>
60</div>
61<div class="section" id="transform-iterator-synopsis">
62<h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> synopsis</a></h1>
63<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
64<!-- Software License, Version 1.0. (See accompanying -->
65<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
66<!-- Version 1.3 of this document was accepted for TR1 -->
67<pre class="literal-block">
68template &lt;class UnaryFunction,
69 class Iterator,
70 class Reference = use_default,
71 class Value = use_default&gt;
72class transform_iterator
73{
74public:
75 typedef /* see below */ value_type;
76 typedef /* see below */ reference;
77 typedef /* see below */ pointer;
78 typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
79 typedef /* see below */ iterator_category;
80
81 transform_iterator();
82 transform_iterator(Iterator const&amp; x, UnaryFunction f);
83
84 template&lt;class F2, class I2, class R2, class V2&gt;
85 transform_iterator(
86 transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
87 , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0 // exposition only
88 , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
89 );
90 UnaryFunction functor() const;
91 Iterator const&amp; base() const;
92 reference operator*() const;
93 transform_iterator&amp; operator++();
94 transform_iterator&amp; operator--();
95private:
96 Iterator m_iterator; // exposition only
97 UnaryFunction m_f; // exposition only
98};
99</pre>
100<p>If <tt class="docutils literal"><span class="pre">Reference</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">reference</span></tt> member of
101<tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is
102<tt class="docutils literal"><span class="pre">result_of&lt;const UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.
103Otherwise, <tt class="docutils literal"><span class="pre">reference</span></tt> is <tt class="docutils literal"><span class="pre">Reference</span></tt>.</p>
104<p>If <tt class="docutils literal"><span class="pre">Value</span></tt> is <tt class="docutils literal"><span class="pre">use_default</span></tt> then the <tt class="docutils literal"><span class="pre">value_type</span></tt> member is
105<tt class="docutils literal"><span class="pre">remove_cv&lt;remove_reference&lt;reference&gt;</span> <span class="pre">&gt;::type</span></tt>. Otherwise,
106<tt class="docutils literal"><span class="pre">value_type</span></tt> is <tt class="docutils literal"><span class="pre">Value</span></tt>.</p>
107<p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and if <tt class="docutils literal"><span class="pre">Iterator</span></tt>
108models Random Access Traversal Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
109convertible to <tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
110<tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator, then
111<tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
112<tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
113convertible to <tt class="docutils literal"><span class="pre">forward_iterator_tag</span></tt>. If <tt class="docutils literal"><span class="pre">Iterator</span></tt> does not
114model Readable Lvalue Iterator then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
115convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
116</div>
117<div class="section" id="transform-iterator-requirements">
118<h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> requirements</a></h1>
119<p>The type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible, and
120the expression <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be valid where <tt class="docutils literal"><span class="pre">f</span></tt> is a const object of
121type <tt class="docutils literal"><span class="pre">UnaryFunction</span></tt>, <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>, and
122where the type of <tt class="docutils literal"><span class="pre">f(*i)</span></tt> must be
123<tt class="docutils literal"><span class="pre">result_of&lt;const UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.</p>
124<p>The argument <tt class="docutils literal"><span class="pre">Iterator</span></tt> shall model Readable Iterator.</p>
125</div>
126<div class="section" id="transform-iterator-models">
127<h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</a></h1>
128<p>The resulting <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
129following that is also modeled by <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
130<blockquote>
131<ul class="simple">
132<li>Writable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a non-const reference.</li>
133<li>Readable Lvalue Iterator if <tt class="docutils literal"><span class="pre">transform_iterator::reference</span></tt> is a const reference.</li>
134<li>Readable Iterator otherwise.</li>
135</ul>
136</blockquote>
137<p>The <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
138concept that is modeled by the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument.</p>
139<p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> is a model of Readable Lvalue Iterator then
140it models the following original iterator concepts depending on what
141the <tt class="docutils literal"><span class="pre">Iterator</span></tt> argument models.</p>
142<table border="1" class="docutils">
143<colgroup>
144<col width="47%" />
145<col width="53%" />
146</colgroup>
147<thead valign="bottom">
148<tr><th class="head">If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models</th>
149<th class="head">then <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models</th>
150</tr>
151</thead>
152<tbody valign="top">
153<tr><td>Single Pass Iterator</td>
154<td>Input Iterator</td>
155</tr>
156<tr><td>Forward Traversal Iterator</td>
157<td>Forward Iterator</td>
158</tr>
159<tr><td>Bidirectional Traversal Iterator</td>
160<td>Bidirectional Iterator</td>
161</tr>
162<tr><td>Random Access Traversal Iterator</td>
163<td>Random Access Iterator</td>
164</tr>
165</tbody>
166</table>
167<p>If <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> models Writable Lvalue Iterator then it is a
168mutable iterator (as defined in the old iterator requirements).</p>
169<p><tt class="docutils literal"><span class="pre">transform_iterator&lt;F1,</span> <span class="pre">X,</span> <span class="pre">R1,</span> <span class="pre">V1&gt;</span></tt> is interoperable with
170<tt class="docutils literal"><span class="pre">transform_iterator&lt;F2,</span> <span class="pre">Y,</span> <span class="pre">R2,</span> <span class="pre">V2&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is
171interoperable with <tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
172</div>
173<div class="section" id="transform-iterator-operations">
174<h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">transform_iterator</span></tt> operations</a></h1>
175<p>In addition to the operations required by the concepts modeled by
176<tt class="docutils literal"><span class="pre">transform_iterator</span></tt>, <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> provides the following
177operations.</p>
178<p><tt class="docutils literal"><span class="pre">transform_iterator();</span></tt></p>
179<table class="docutils field-list" frame="void" rules="none">
180<col class="field-name" />
181<col class="field-body" />
182<tbody valign="top">
183<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
184and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
185</tr>
186</tbody>
187</table>
188<p><tt class="docutils literal"><span class="pre">transform_iterator(Iterator</span> <span class="pre">const&amp;</span> <span class="pre">x,</span> <span class="pre">UnaryFunction</span> <span class="pre">f);</span></tt></p>
189<table class="docutils field-list" frame="void" rules="none">
190<col class="field-name" />
191<col class="field-body" />
192<tbody valign="top">
193<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
194initialized to <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
195</tr>
196</tbody>
197</table>
198<pre class="literal-block">
199template&lt;class F2, class I2, class R2, class V2&gt;
200transform_iterator(
201 transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
202 , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0 // exposition only
203 , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition only
204);
205</pre>
206<table class="docutils field-list" frame="void" rules="none">
207<col class="field-name" />
208<col class="field-body" />
209<tbody valign="top">
210<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
211initialized to <tt class="docutils literal"><span class="pre">t.functor()</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to
212<tt class="docutils literal"><span class="pre">t.base()</span></tt>.</td>
213</tr>
214<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
215</tr>
216</tbody>
217</table>
218<p><tt class="docutils literal"><span class="pre">UnaryFunction</span> <span class="pre">functor()</span> <span class="pre">const;</span></tt></p>
219<table class="docutils field-list" frame="void" rules="none">
220<col class="field-name" />
221<col class="field-body" />
222<tbody valign="top">
223<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f</span></tt></td>
224</tr>
225</tbody>
226</table>
227<p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
228<table class="docutils field-list" frame="void" rules="none">
229<col class="field-name" />
230<col class="field-body" />
231<tbody valign="top">
232<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>
233</tr>
234</tbody>
235</table>
236<p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
237<table class="docutils field-list" frame="void" rules="none">
238<col class="field-name" />
239<col class="field-body" />
240<tbody valign="top">
241<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_f(*m_iterator)</span></tt></td>
242</tr>
243</tbody>
244</table>
245<p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
246<table class="docutils field-list" frame="void" rules="none">
247<col class="field-name" />
248<col class="field-body" />
249<tbody valign="top">
250<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>
251</tr>
252<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
253</tr>
254</tbody>
255</table>
256<p><tt class="docutils literal"><span class="pre">transform_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
257<table class="docutils field-list" frame="void" rules="none">
258<col class="field-name" />
259<col class="field-body" />
260<tbody valign="top">
261<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>
262</tr>
263<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
264</tr>
265</tbody>
266</table>
267<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
268<!-- Software License, Version 1.0. (See accompanying -->
269<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
270<pre class="literal-block">
271template &lt;class UnaryFunction, class Iterator&gt;
272transform_iterator&lt;UnaryFunction, Iterator&gt;
273make_transform_iterator(Iterator it, UnaryFunction fun);
274</pre>
275<table class="docutils field-list" frame="void" rules="none">
276<col class="field-name" />
277<col class="field-body" />
278<tbody valign="top">
279<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator&lt;UnaryFunction,</span> <span class="pre">Iterator&gt;</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
280initialized to <tt class="docutils literal"><span class="pre">f</span></tt> and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
281</tr>
282</tbody>
283</table>
284<pre class="literal-block">
285template &lt;class UnaryFunction, class Iterator&gt;
286transform_iterator&lt;UnaryFunction, Iterator&gt;
287make_transform_iterator(Iterator it);
288</pre>
289<table class="docutils field-list" frame="void" rules="none">
290<col class="field-name" />
291<col class="field-body" />
292<tbody valign="top">
293<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">transform_iterator&lt;UnaryFunction,</span> <span class="pre">Iterator&gt;</span></tt> with <tt class="docutils literal"><span class="pre">m_f</span></tt>
294default constructed and <tt class="docutils literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
295</tr>
296</tbody>
297</table>
298<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
299<!-- Software License, Version 1.0. (See accompanying -->
300<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
301</div>
302<div class="section" id="example">
303<h1><a class="toc-backref" href="#id6">Example</a></h1>
304<p>This is a simple example of using the transform_iterators class to
305generate iterators that multiply (or add to) the value returned by
306dereferencing the iterator. It would be cooler to use lambda library
307in this example.</p>
308<pre class="literal-block">
309int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
310const int N = sizeof(x)/sizeof(int);
311
312typedef boost::binder1st&lt; std::multiplies&lt;int&gt; &gt; Function;
313typedef boost::transform_iterator&lt;Function, int*&gt; doubling_iterator;
314
315doubling_iterator i(x, boost::bind1st(std::multiplies&lt;int&gt;(), 2)),
316 i_end(x + N, boost::bind1st(std::multiplies&lt;int&gt;(), 2));
317
318std::cout &lt;&lt; &quot;multiplying the array by 2:&quot; &lt;&lt; std::endl;
319while (i != i_end)
320 std::cout &lt;&lt; *i++ &lt;&lt; &quot; &quot;;
321std::cout &lt;&lt; std::endl;
322
323std::cout &lt;&lt; &quot;adding 4 to each element in the array:&quot; &lt;&lt; std::endl;
324std::copy(boost::make_transform_iterator(x, boost::bind1st(std::plus&lt;int&gt;(), 4)),
325 boost::make_transform_iterator(x + N, boost::bind1st(std::plus&lt;int&gt;(), 4)),
326 std::ostream_iterator&lt;int&gt;(std::cout, &quot; &quot;));
327std::cout &lt;&lt; std::endl;
328</pre>
329<p>The output is:</p>
330<pre class="literal-block">
331multiplying the array by 2:
3322 4 6 8 10 12 14 16
333adding 4 to each element in the array:
3345 6 7 8 9 10 11 12
335</pre>
336<p>The source code for this example can be found <a class="reference external" href="../example/transform_iterator_example.cpp">here</a>.</p>
337</div>
338</div>
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