<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Pitfalls</title> <link rel="stylesheet" href="../boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../foreach.html" title="Chapter 6. Boost.Foreach"> <link rel="prev" href="portability.html" title="Portability"> <link rel="next" href="history_and_acknowledgements.html" title="History and Acknowledgements"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="portability.html"><img src="../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../foreach.html"><img src="../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="history_and_acknowledgements.html"><img src="../../../doc/html/images/next.png" alt="Next"></a> </div> <div class="section" title="Pitfalls"> <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="id864044"></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="id864496"></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> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="portability.html"><img src="../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../foreach.html"><img src="../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="history_and_acknowledgements.html"><img src="../../../doc/html/images/next.png" alt="Next"></a> </div> </body> </html>