forked from boostorg/range
[boost][range] - Update to relax preconditions for the strided adaptor, and numerous fixes to inspection report issues.
[SVN r67418]
This commit is contained in:
Neil Groves
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<title>Introduction and motivation</title>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="range.reference.algorithms.introduction"></a><a class="link" href="introduction.html" title="Introduction and motivation">Introduction
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and motivation</a>
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</h4></div></div></div>
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<p>
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In its most simple form a <span class="bold"><strong>Range Algorithm</strong></span>
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(or range-based algorithm) is simply an iterator-based algorithm where
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the <span class="emphasis"><em>two</em></span> iterator arguments have been replaced by
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<span class="emphasis"><em>one</em></span> range argument. For example, we may write
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</p>
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<p>
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</p>
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<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">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span>
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<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="special">...;</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">);</span>
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</pre>
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<p>
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</p>
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<p>
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instead of
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
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</pre>
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<p>
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</p>
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<p>
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However, the return type of range algorithms is almost always different
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from that of existing iterator-based algorithms.
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</p>
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<p>
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One group of algorithms, like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">()</span></code>, will simply return the same range so
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that we can continue to pass the range around and/or further modify it.
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Because of this we may write
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</p>
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<pre class="programlisting"><span class="identifier">boost</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">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">));</span>
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</pre>
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<p>
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to first sort the range and then run <code class="computeroutput"><span class="identifier">unique</span><span class="special">()</span></code> on the sorted range.
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</p>
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<p>
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Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">()</span></code>
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fall into another group of algorithms that return (potentially) narrowed
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views of the original range. By default <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">rng</span><span class="special">)</span></code> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">found</span><span class="special">)</span></code>
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where <code class="computeroutput"><span class="identifier">found</span></code> denotes the
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iterator returned by <code class="computeroutput"><span class="identifier">std</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">begin</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">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code>
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</p>
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<p>
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Therefore exactly the unique values can be copied by writing
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</p>
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<pre class="programlisting"><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">unique</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">)),</span>
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<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>
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</pre>
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<p>
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</p>
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<p>
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Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span></code> usually return the same range:
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<code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">found</span><span class="special">)</span></code>. However, this behaviour may be changed
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by supplying the algorithms with a template argument:
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Expression
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</p>
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</th>
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<th>
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<p>
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Return
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_found</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns a single iterator like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_begin_found</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span>
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<span class="identifier">found</span><span class="special">)</span></code>
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(this is the default)
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_begin_next</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">found</span><span class="special">))</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_found_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">found</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_next_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">found</span><span class="special">),</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code>
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">boost</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">return_begin_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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returns the entire original range.
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<p>
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This functionality has the following advantages:
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</p>
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<div class="orderedlist"><ol class="orderedlist" type="1">
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<li class="listitem">
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it allows for <span class="emphasis"><em><span class="bold"><strong>seamless functional-style
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programming</strong></span></em></span> where you do not need to use named
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local variables to store intermediate results
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</li>
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<li class="listitem">
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it is very <span class="emphasis"><em><span class="bold"><strong>safe</strong></span></em></span>
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because the algorithm can verify out-of-bounds conditions and handle
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tricky conditions that lead to empty ranges
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</li>
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</ol></div>
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<p>
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For example, consider how easy we may erase the duplicates in a sorted
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container:
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</p>
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<p>
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</p>
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<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="special">...;</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">erase</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">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">)));</span>
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</pre>
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<p>
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</p>
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<p>
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Notice the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span></code>.
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What if we wanted to erase all the duplicates except one of them? In old-fashined
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STL-programming we might write
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="comment">// assume 'vec' is already sorted
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</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">iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">vec</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
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<span class="comment">// remember this check or you get into problems
|
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</span><span class="keyword">if</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">!=</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">())</span>
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<span class="special">++</span><span class="identifier">i</span><span class="special">;</span>
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<span class="identifier">vec</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
|
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</pre>
|
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<p>
|
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</p>
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<p>
|
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The same task may be accomplished simply with
|
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</p>
|
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<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">erase</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">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_next_end</span><span class="special">>(</span><span class="identifier">vec</span><span class="special">));</span>
|
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</pre>
|
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<p>
|
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and there is no need to worry about generating an invalid range. Furthermore,
|
||||
if the container is complex, calling <code class="computeroutput"><span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span></code> several times will be more expensive
|
||||
than using a range algorithm.
|
||||
</p>
|
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</div>
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||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<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. (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>)
|
||||
</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../algorithms.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../algorithms.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="mutating.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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