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<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>
<h3>
<a name="foreach.pitfalls.h0"></a>
<span class="phrase"><a name="foreach.pitfalls.types_with_commas"></a></span><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>
<h3>
<a name="foreach.pitfalls.h1"></a>
<span class="phrase"><a name="foreach.pitfalls.hoisting_and_iterator_invalidation"></a></span><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>
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<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>
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