<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Introduction and motivation</title> <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../../../index.html" title="Chapter 1. Range 2.0"> <link rel="up" href="../adaptors.html" title="Range Adaptors"> <link rel="prev" href="../adaptors.html" title="Range Adaptors"> <link rel="next" href="general_requirements.html" title="General Requirements"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../adaptors.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../adaptors.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="general_requirements.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <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> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2003 -2010 Thorsten Ottosen, Neil Groves<p> Distributed under the Boost Software License, Version 1.0. 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