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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="foreach.pitfalls"></a><a class="link" href="pitfalls.html" title="Pitfalls">Pitfalls</a> |
| </h2></div></div></div> |
| <p> |
| This section describes some common pitfalls with <code class="literal">BOOST_FOREACH</code>. |
| </p> |
| <a name="foreach.pitfalls.types_with_commas"></a><h3> |
| <a name="id1262295"></a> |
| <a class="link" href="pitfalls.html#foreach.pitfalls.types_with_commas">Types With Commas</a> |
| </h3> |
| <p> |
| Since <code class="literal">BOOST_FOREACH</code> is a macro, it must have exactly two |
| arguments, with exactly one comma separating them. That's not always convenient, |
| especially when the type of the loop variable is a template. Consider trying |
| to iterate over a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>: |
| </p> |
| <pre class="programlisting"><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> |
| |
| <span class="comment">// ERROR! Too many arguments to BOOST_FOREACH macro. |
| </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">// ... |
| </span></pre> |
| <p> |
| One way to fix this is with a typedef. |
| </p> |
| <pre class="programlisting"><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> |
| <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> |
| |
| <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">// ... |
| </span></pre> |
| <p> |
| Another way to fix it is to predeclare the loop variable: |
| </p> |
| <pre class="programlisting"><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> |
| <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">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">// ... |
| </span></pre> |
| <a name="foreach.pitfalls.hoisting_and_iterator_invalidation"></a><h3> |
| <a name="id1262747"></a> |
| <a class="link" href="pitfalls.html#foreach.pitfalls.hoisting_and_iterator_invalidation">Hoisting |
| and Iterator Invalidation</a> |
| </h3> |
| <p> |
| Under the covers, <code class="literal">BOOST_FOREACH</code> uses iterators to traverse |
| the element sequence. Before the loop is executed, the end iterator is cached |
| in a local variable. This is called <span class="emphasis"><em>hoisting</em></span>, and it is |
| an important optimization. It assumes, however, that the end iterator of the |
| sequence is stable. It usually is, but if we modify the sequence by adding |
| or removing elements while we are iterating over it, we may end up hoisting |
| ourselves on our own petard. |
| </p> |
| <p> |
| Consider the following code: |
| </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">vect</span><span class="special">(</span><span class="number">4</span><span class="special">,</span> <span class="number">4</span><span class="special">);</span> |
| <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> |
| <span class="special">{</span> |
| <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="special">}</span> |
| </pre> |
| <p> |
| This code will compile, but it has undefined behavior. That is because it is |
| logically equivalent to the following: |
| </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">vect</span><span class="special">(</span><span class="number">4</span><span class="special">,</span> <span class="number">4</span><span class="special">);</span> |
| <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> |
| <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> |
| <span class="special">{</span> |
| <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> |
| <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! |
| </span><span class="special">}</span> |
| </pre> |
| <p> |
| 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> |
| will cause all iterators into <code class="computeroutput"><span class="identifier">vect</span></code> |
| to become invalid, including <code class="computeroutput"><span class="identifier">it1</span></code> |
| and <code class="computeroutput"><span class="identifier">it2</span></code>. The next iteration |
| through the loop will cause the invalid iterators to be used. That's bad news. |
| </p> |
| <p> |
| The moral of the story is to think twice before adding and removing elements |
| from the sequence over which you are iterating. If doing so could cause iterators |
| to become invalid, don't do it. Use a regular <code class="computeroutput"><span class="keyword">for</span></code> |
| loop instead. |
| </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 © 2004 Eric Niebler<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> |
| </div></td> |
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