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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="algorithm.CXX14"></a><a class="link" href="CXX14.html" title="C++14 Algorithms">C++14 Algorithms</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <dt><span class="section"><a href="CXX14.html#the_boost_algorithm_library.CXX14.equal">equal </a></span></dt> |
| <dt><span class="section"><a href="../the_boost_algorithm_library/CXX14/mismatch.html">mismatch |
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| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="the_boost_algorithm_library.CXX14.equal"></a><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal" title="equal">equal </a> |
| </h3></div></div></div> |
| <p> |
| The header file 'equal.hpp' contains two variants of a the stl algorithm |
| <code class="computeroutput"><span class="identifier">equal</span></code>. The algorithm tests |
| to see if two sequences contain equal values; |
| </p> |
| <p> |
| Before (the proposed) C++14 the algorithm <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">equal</span></code> |
| took three iterators and an optional comparison predicate. The first two |
| iterators <code class="computeroutput"><span class="special">[</span><span class="identifier">first1</span><span class="special">,</span> <span class="identifier">last1</span><span class="special">)</span></code> defined a sequence, and the second one |
| <code class="computeroutput"><span class="identifier">first2</span></code> defined the start |
| of the second sequence. The second sequence was assumed to be the same length |
| as the first. |
| </p> |
| <p> |
| In C++14, two new variants were introduced, taking four iterators and an |
| optional comparison predicate. The four iterators define two sequences <code class="computeroutput"><span class="special">[</span><span class="identifier">first1</span><span class="special">,</span> <span class="identifier">last1</span><span class="special">)</span></code> and <code class="computeroutput"><span class="special">[</span><span class="identifier">first2</span><span class="special">,</span> <span class="identifier">last2</span><span class="special">)</span></code> |
| explicitly, rather than defining the second one implicitly. This leads to |
| correct answers in more cases (and avoid undefined behavior in others). |
| </p> |
| <p> |
| Consider the two sequences: |
| </p> |
| <pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">seq1</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">2</span> <span class="special">};</span> |
| <span class="keyword">auto</span> <span class="identifier">seq2</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">4</span> <span class="special">};</span> |
| |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">seq1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">seq1</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">seq2</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">());</span> <span class="comment">// true</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">seq2</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">seq2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">seq1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">());</span> <span class="comment">// Undefined behavior</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">seq1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">seq1</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">seq2</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">seq2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">());</span> <span class="comment">// false</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| You can argue that <code class="computeroutput"><span class="keyword">true</span></code> is the |
| correct answer in the first case, even though the sequences are not the same. |
| The first N entries in <code class="computeroutput"><span class="identifier">seq2</span></code> |
| are the same as the entries in <code class="computeroutput"><span class="identifier">seq1</span></code> |
| - but that's not all that's in <code class="computeroutput"><span class="identifier">seq2</span></code>. |
| But in the second case, the algorithm will read past the end of <code class="computeroutput"><span class="identifier">seq1</span></code>, resulting in undefined behavior (large |
| earthquake, incorrect results, pregnant cat, etc). |
| </p> |
| <p> |
| However, if the two sequences are specified completely, it's clear that they |
| are not equal. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h0"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.interface"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.interface">interface</a> |
| </h5> |
| <p> |
| The function <code class="computeroutput"><span class="identifier">equal</span></code> returns |
| true if the two sequences compare equal; i.e, if each element in the sequence |
| compares equal to the corresponding element in the other sequence. One version |
| uses <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span></code> to do the comparison; the other |
| lets the caller pass predicate to do the comparisons. |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">InputIterator1</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">InputIterator2</span><span class="special">></span> |
| <span class="keyword">bool</span> <span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">InputIterator1</span> <span class="identifier">first1</span><span class="special">,</span> <span class="identifier">InputIterator1</span> <span class="identifier">last1</span><span class="special">,</span> |
| <span class="identifier">InputIterator2</span> <span class="identifier">first2</span><span class="special">,</span> <span class="identifier">InputIterator2</span> <span class="identifier">last2</span> <span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">InputIterator1</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">InputIterator2</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">BinaryPredicate</span><span class="special">></span> |
| <span class="keyword">bool</span> <span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">InputIterator1</span> <span class="identifier">first1</span><span class="special">,</span> <span class="identifier">InputIterator1</span> <span class="identifier">last1</span><span class="special">,</span> |
| <span class="identifier">InputIterator2</span> <span class="identifier">first2</span><span class="special">,</span> <span class="identifier">InputIterator2</span> <span class="identifier">last2</span><span class="special">,</span> <span class="identifier">BinaryPredicate</span> <span class="identifier">pred</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h1"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.examples"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.examples">Examples</a> |
| </h5> |
| <p> |
| Given the container <code class="computeroutput"><span class="identifier">c1</span></code> containing |
| <code class="computeroutput"><span class="special">{</span> <span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> |
| <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">14</span><span class="special">,</span> <span class="number">15</span> <span class="special">}</span></code>, |
| and <code class="computeroutput"><span class="identifier">c2</span></code> containing <code class="computeroutput"><span class="special">{</span> <span class="number">1</span><span class="special">,</span> |
| <span class="number">2</span><span class="special">,</span> <span class="number">3</span> <span class="special">}</span></code>, then |
| </p> |
| <pre class="programlisting"><span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">())</span> <span class="special">--></span> <span class="keyword">false</span> |
| <span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">()</span> <span class="special">+</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">()</span> <span class="special">+</span> <span class="number">3</span><span class="special">,</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">begin</span> <span class="special">(),</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">())</span> <span class="special">--></span> <span class="keyword">true</span> |
| <span class="identifier">equal</span> <span class="special">(</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">c1</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">(),</span> <span class="identifier">c2</span><span class="special">.</span><span class="identifier">end</span> <span class="special">())</span> <span class="special">--></span> <span class="keyword">true</span> <span class="comment">// empty sequences are alway equal to each other</span> |
| </pre> |
| <p> |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h2"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.iterator_requirements"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.iterator_requirements">Iterator |
| Requirements</a> |
| </h5> |
| <p> |
| <code class="computeroutput"><span class="identifier">equal</span></code> works on all iterators |
| except output iterators. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h3"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.complexity"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.complexity">Complexity</a> |
| </h5> |
| <p> |
| Both of the variants of <code class="computeroutput"><span class="identifier">equal</span></code> |
| run in <span class="emphasis"><em>O(N)</em></span> (linear) time; that is, they compare against |
| each element in the list once. If the sequence is found to be not equal at |
| any point, the routine will terminate immediately, without examining the |
| rest of the elements. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h4"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.exception_safety"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.exception_safety">Exception |
| Safety</a> |
| </h5> |
| <p> |
| Both of the variants of <code class="computeroutput"><span class="identifier">equal</span></code> |
| take their parameters by value and do not depend upon any global state. Therefore, |
| all the routines in this file provide the strong exception guarantee. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.CXX14.equal.h5"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.CXX14.equal.notes"></a></span><a class="link" href="CXX14.html#the_boost_algorithm_library.CXX14.equal.notes">Notes</a> |
| </h5> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| The four iterator version of the routine <code class="computeroutput"><span class="identifier">equal</span></code> |
| is part of the C++14 standard. When C++14 standard library implementations |
| become available, the implementation from the standard library should |
| be used. |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">equal</span></code> returns true for |
| two empty ranges, no matter what predicate is passed to test against. |
| </li> |
| </ul></div> |
| </div> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2010-2012 Marshall Clow<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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