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| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 27 | <a name="foreach.pitfalls"></a>Pitfalls</h2></div></div></div> |
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| 28 | <p> |
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| 29 | This section describes some common pitfalls with <code class="literal">BOOST_FOREACH</code>. |
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| 30 | </p> |
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| 31 | <a name="foreach.pitfalls.types_with_commas"></a><h2> |
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| 32 | <a name="id1184829"></a> |
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| 33 | Types With Commas |
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| 34 | </h2> |
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| 35 | <p> |
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| 36 | Since <code class="literal">BOOST_FOREACH</code> is a macro, it must have exactly two |
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| 37 | arguments, with exactly one comma separating them. That's not always convenient, |
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| 38 | especially when the type of the loop variable is a template. Consider trying |
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| 39 | to iterate over a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>: |
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| 40 | </p> |
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| 41 | <pre class="programlisting"> |
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| 42 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">m</span><span class="special">;</span> |
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| 43 | |
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| 44 | <span class="comment">// ERROR! Too many arguments to BOOST_FOREACH macro. |
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| 45 | </span><span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">m</span><span class="special">)</span> <span class="comment">// ... |
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| 46 | </span></pre> |
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| 47 | <p> |
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| 48 | One way to fix this is with a typedef. |
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| 49 | </p> |
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| 50 | <pre class="programlisting"> |
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| 51 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">m</span><span class="special">;</span> |
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| 52 | <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">pair_t</span><span class="special">;</span> |
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| 53 | |
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| 54 | <span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">pair_t</span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">m</span><span class="special">)</span> <span class="comment">// ... |
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| 55 | </span></pre> |
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| 56 | <p> |
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| 57 | Another way to fix it is to predeclare the loop variable: |
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| 58 | </p> |
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| 59 | <pre class="programlisting"> |
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| 60 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">m</span><span class="special">;</span> |
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| 61 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">p</span><span class="special">;</span> |
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| 62 | |
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| 63 | <span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">m</span><span class="special">)</span> <span class="comment">// ... |
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| 64 | </span></pre> |
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| 65 | <a name="foreach.pitfalls.hoisting_and_iterator_invalidation"></a><h2> |
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| 66 | <a name="id1185275"></a> |
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| 67 | Hoisting and Iterator Invalidation |
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| 68 | </h2> |
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| 69 | <p> |
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| 70 | Under the covers, <code class="literal">BOOST_FOREACH</code> uses iterators to traverse |
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| 71 | the element sequence. Before the loop is executed, the end iterator is cached |
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| 72 | in a local variable. This is called <span class="emphasis"><em>hoisting</em></span>, and it is |
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| 73 | an important optimization. It assumes, however, that the end iterator of the |
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| 74 | sequence is stable. It usually is, but if we modify the sequence by adding |
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| 75 | or removing elements while we are iterating over it, we may end up hoisting |
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| 76 | ourselves on our own petard. |
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| 77 | </p> |
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| 78 | <p> |
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| 79 | Consider the following code: |
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| 80 | </p> |
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| 81 | <pre class="programlisting"> |
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| 82 | <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">vect</span><span class="special">(</span><span class="number">4</span><span class="special">,</span> <span class="number">4</span><span class="special">);</span> |
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| 83 | <span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">vect</span><span class="special">)</span> |
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| 84 | <span class="special">{</span> |
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| 85 | <span class="identifier">vect</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> |
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| 86 | <span class="special">}</span> |
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| 87 | </pre> |
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| 88 | <p> |
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| 89 | This code will compile, but it has undefined behavior. That is because it is |
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| 90 | logically equivalent to the following: |
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| 91 | </p> |
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| 92 | <pre class="programlisting"> |
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| 93 | <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">vect</span><span class="special">(</span><span class="number">4</span><span class="special">,</span> <span class="number">4</span><span class="special">);</span> |
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| 94 | <span class="keyword">for</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">iterator</span> <span class="identifier">it1</span> <span class="special">=</span> <span class="identifier">vect</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">it2</span> <span class="special">=</span> <span class="identifier">vect</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span> |
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| 95 | <span class="identifier">it1</span> <span class="special">!=</span> <span class="identifier">it2</span><span class="special">;</span> <span class="special">++</span><span class="identifier">it1</span><span class="special">)</span> |
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| 96 | <span class="special">{</span> |
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| 97 | <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">it1</span><span class="special">;</span> |
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| 98 | <span class="identifier">vect</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> <span class="comment">// Oops! This invalidates it1 and it2! |
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| 99 | </span><span class="special">}</span> |
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| 100 | </pre> |
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| 101 | <p> |
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| 102 | The call to <code class="computeroutput"><span class="identifier">vect</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">()</span></code> |
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| 103 | will cause all iterators into <code class="computeroutput"><span class="identifier">vect</span></code> |
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| 104 | to become invalid, including <code class="computeroutput"><span class="identifier">it1</span></code> |
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| 105 | and <code class="computeroutput"><span class="identifier">it2</span></code>. The next iteration |
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| 106 | through the loop will cause the invalid iterators to be used. That's bad news. |
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| 107 | </p> |
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| 108 | <p> |
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| 109 | The moral of the story is to think twice before adding and removing elements |
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| 110 | from the sequence over which you are iterating. If doing so could cause iterators |
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| 111 | to become invalid, don't do it. Use a regular <code class="computeroutput"><span class="keyword">for</span></code> |
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| 112 | loop instead. |
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| 113 | </p> |
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| 114 | </div> |
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| 116 | <td align="left"></td> |
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| 117 | <td align="right"><small>Copyright © 2004 Eric Niebler</small></td> |
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