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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="range.reference.adaptors.introduction"></a><a class="link" href="introduction.html" title="Introduction and motivation"> Introduction |
| and motivation</a> |
| </h4></div></div></div> |
| <p> |
| A <span class="bold"><strong>Range Adaptor</strong></span> is a class that wraps |
| an existing Range to provide a new Range with different behaviour. Since |
| the behaviour of Ranges is determined by their associated iterators, a |
| Range Adaptor simply wraps the underlying iterators with new special iterators. |
| In this example |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">range</span><span class="special">/</span><span class="identifier">adaptors</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">range</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> |
| |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span><span class="special">,</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| the iterators from <code class="computeroutput"><span class="identifier">vec</span></code> |
| are wrapped <code class="computeroutput"><span class="identifier">reverse_iterator</span></code>s. |
| The type of the underlying Range Adapter is not documented because you |
| do not need to know it. All that is relevant is that the expression |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| returns a Range Adaptor where the iterator type is now the iterator type |
| of the range <code class="computeroutput"><span class="identifier">vec</span></code> wrapped |
| in <code class="computeroutput"><span class="identifier">reverse_iterator</span></code>. The |
| expression <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span></code> is called an <span class="bold"><strong>Adaptor |
| Generator</strong></span>. |
| </p> |
| <p> |
| There are two ways of constructing a range adaptor. The first is by using |
| <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>. |
| This is my preferred technique, however while discussing range adaptors |
| with others it became clear that some users of the library strongly prefer |
| a more familiar function syntax, so equivalent functions of the present |
| tense form have been added as an alternative syntax. The equivalent to |
| <code class="computeroutput"><span class="identifier">rng</span> <span class="special">|</span> |
| <span class="identifier">reversed</span></code> is <code class="computeroutput"><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span><span class="identifier">rng</span><span class="special">)</span></code> for example. |
| </p> |
| <p> |
| Why do I prefer the <code class="computeroutput"><span class="keyword">operator</span><span class="special">|</span></code> syntax? The answer is readability: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span><span class="identifier">vec</span><span class="special">),</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| This might not look so bad, but when we apply several adaptors, it becomes |
| much worse. Just compare |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">)</span> <span class="special">),</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| to |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span> |
| <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">uniqued</span><span class="special">,</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Furthermore, some of the adaptor generators take arguments themselves and |
| these arguments are expressed with function call notation too. In those |
| situations, you will really appreciate the succinctness of <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>. |
| </p> |
| <a name="range.reference.adaptors.introduction.composition_of_adaptors"></a><h6> |
| <a name="id771349"></a> |
| <a class="link" href="introduction.html#range.reference.adaptors.introduction.composition_of_adaptors">Composition |
| of Adaptors</a> |
| </h6> |
| <p> |
| Range Adaptors are a powerful complement to Range algorithms. The reason |
| is that adaptors are <span class="emphasis"><em><span class="bold"><strong>orthogonal</strong></span></em></span> |
| to algorithms. For example, consider these Range algorithms: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">out</span> <span class="special">)</span></code> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count</span><span class="special">(</span> |
| <span class="identifier">rng</span><span class="special">,</span> |
| <span class="identifier">pred</span> <span class="special">)</span></code> |
| </li> |
| </ul></div> |
| <p> |
| What should we do if we only want to copy an element <code class="computeroutput"><span class="identifier">a</span></code> |
| if it satisfies some predicate, say <code class="computeroutput"><span class="identifier">pred</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>? |
| And what if we only want to count the elements that satisfy the same predicate? |
| The naive answer would be to use these algorithms: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy_if</span><span class="special">(</span> |
| <span class="identifier">rng</span><span class="special">,</span> |
| <span class="identifier">pred</span><span class="special">,</span> |
| <span class="identifier">out</span> <span class="special">)</span></code> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count_if</span><span class="special">(</span> |
| <span class="identifier">rng</span><span class="special">,</span> |
| <span class="identifier">pred</span> <span class="special">)</span></code> |
| </li> |
| </ul></div> |
| <p> |
| These algorithms are only defined to maintain a one to one relationship |
| with the standard library algorithms. This approach of adding algorithm |
| suffers a combinatorial explosion. Inevitably many algorithms are missing |
| <code class="computeroutput"><span class="identifier">_if</span></code> variants and there |
| is redundant development overhead for each new algorithm. The Adaptor Generator |
| is the design solution to this problem. |
| </p> |
| <a name="range.reference.adaptors.introduction.range_adaptor_alternative_to_copy_if_algorithm"></a><h6> |
| <a name="id771666"></a> |
| <a class="link" href="introduction.html#range.reference.adaptors.introduction.range_adaptor_alternative_to_copy_if_algorithm">Range |
| Adaptor alternative to copy_if algorithm</a> |
| </h6> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy_if</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">pred</span><span class="special">,</span> <span class="identifier">out</span> <span class="special">);</span> |
| </pre> |
| <p> |
| can be expressed as |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">filtered</span><span class="special">(</span><span class="identifier">pred</span><span class="special">),</span> <span class="identifier">out</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <a name="range.reference.adaptors.introduction.range_adaptor_alternative_to_count_if_algorithm"></a><h6> |
| <a name="id771832"></a> |
| <a class="link" href="introduction.html#range.reference.adaptors.introduction.range_adaptor_alternative_to_count_if_algorithm">Range |
| Adaptor alternative to count_if algorithm</a> |
| </h6> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count_if</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">pred</span> <span class="special">);</span> |
| </pre> |
| <p> |
| can be expressed as |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count</span><span class="special">(</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">filtered</span><span class="special">(</span><span class="identifier">pred</span><span class="special">),</span> <span class="identifier">out</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| What this means is that <span class="emphasis"><em><span class="bold"><strong>no</strong></span></em></span> |
| algorithm with the <code class="computeroutput"><span class="identifier">_if</span></code> |
| suffix is needed. Furthermore, it turns out that algorithms with the <code class="computeroutput"><span class="identifier">_copy</span></code> suffix are not needed either. Consider |
| the somewhat misdesigned <code class="computeroutput"><span class="identifier">replace_copy_if</span><span class="special">()</span></code> which may be used as |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">replace_copy_if</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">back_inserter</span><span class="special">(</span><span class="identifier">vec</span><span class="special">),</span> <span class="identifier">pred</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| With adaptors and algorithms we can express this as |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span><span class="special">;</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">replaced_if</span><span class="special">(</span><span class="identifier">pred</span><span class="special">,</span> <span class="identifier">new_value</span><span class="special">));</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The latter code has several benefits: |
| </p> |
| <p> |
| 1. it is more <span class="emphasis"><em><span class="bold"><strong>efficient</strong></span></em></span> |
| because we avoid extra allocations as might happen with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">back_inserter</span></code> |
| </p> |
| <p> |
| 2. it is <span class="emphasis"><em><span class="bold"><strong>flexible</strong></span></em></span> |
| as we can subsequently apply even more adaptors, for example: |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">replaced_if</span><span class="special">(</span><span class="identifier">pred</span><span class="special">,</span> <span class="identifier">new_value</span><span class="special">)</span> |
| <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| 3. it is <span class="emphasis"><em><span class="bold"><strong>safer</strong></span></em></span> because |
| there is no use of an unbounded output iterator. |
| </p> |
| <p> |
| In this manner, the <span class="emphasis"><em><span class="bold"><strong>composition</strong></span></em></span> |
| of Range Adaptors has the following consequences: |
| </p> |
| <p> |
| 1. we no longer need <code class="computeroutput"><span class="identifier">_if</span></code>, |
| <code class="computeroutput"><span class="identifier">_copy</span></code>, <code class="computeroutput"><span class="identifier">_copy_if</span></code> |
| and <code class="computeroutput"><span class="identifier">_n</span></code> variants of algorithms. |
| </p> |
| <p> |
| 2. we can generate a multitude of new algorithms on the fly, for example, |
| above we generated <code class="computeroutput"><span class="identifier">reverse_replace_copy_if</span><span class="special">()</span></code> |
| </p> |
| <p> |
| In other words: |
| </p> |
| <p> |
| <span class="bold"><strong>Range Adaptors are to algorithms what algorithms |
| are to containers</strong></span> |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003 -2010 Thorsten Ottosen, Neil Groves<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
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