1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Chapter 15. Boost.Foreach</title> 6<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> 10<link rel="prev" href="boost_dll/acknowledgements.html" title="Acknowledgements"> 11<link rel="next" href="foreach/extensibility.html" title="Extensibility"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td> 16<td align="center"><a href="../../index.html">Home</a></td> 17<td align="center"><a href="../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="boost_dll/acknowledgements.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="foreach/extensibility.html"><img src="../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="chapter"> 27<div class="titlepage"><div> 28<div><h2 class="title"> 29<a name="foreach"></a>Chapter 15. Boost.Foreach</h2></div> 30<div><div class="author"><h3 class="author"> 31<span class="firstname">Eric</span> <span class="surname">Niebler</span> 32</h3></div></div> 33<div><p class="copyright">Copyright © 2004 Eric Niebler</p></div> 34<div><div class="legalnotice"> 35<a name="foreach.legal"></a><p> 36 Distributed under the Boost Software License, Version 1.0. (See accompanying 37 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>) 38 </p> 39</div></div> 40</div></div> 41<div class="toc"> 42<p><b>Table of Contents</b></p> 43<dl class="toc"> 44<dt><span class="section"><a href="foreach.html#foreach.introduction">Introduction</a></span></dt> 45<dt><span class="section"><a href="foreach/extensibility.html">Extensibility</a></span></dt> 46<dt><span class="section"><a href="foreach/portability.html">Portability</a></span></dt> 47<dt><span class="section"><a href="foreach/pitfalls.html">Pitfalls</a></span></dt> 48<dt><span class="section"><a href="foreach/history_and_acknowledgements.html">History and Acknowledgements</a></span></dt> 49</dl> 50</div> 51<div class="section"> 52<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 53<a name="foreach.introduction"></a><a class="link" href="foreach.html#foreach.introduction" title="Introduction">Introduction</a> 54</h2></div></div></div> 55<div class="blockquote"><blockquote class="blockquote"><p> 56 <span class="quote">“<span class="quote">Make simple things easy.</span>”</span><br> <span class="bold"><strong><span class="emphasis"><em>-- 57 Larry Wall</em></span></strong></span> 58 </p></blockquote></div> 59<h3> 60<a name="foreach.introduction.h0"></a> 61 <span class="phrase"><a name="foreach.introduction.what_is__literal_boost_foreach__literal__"></a></span><a class="link" href="foreach.html#foreach.introduction.what_is__literal_boost_foreach__literal__">What 62 is <code class="literal">BOOST_FOREACH</code>?</a> 63 </h3> 64<p> 65 In C++, writing a loop that iterates over a sequence is tedious. We can either 66 use iterators, which requires a considerable amount of boiler-plate, or we 67 can use the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code> 68 algorithm and move our loop body into a predicate, which requires no less boiler-plate 69 and forces us to move our logic far from where it will be used. In contrast, 70 some other languages, like Perl, provide a dedicated "foreach" construct 71 that automates this process. <code class="literal">BOOST_FOREACH</code> is just such 72 a construct for C++. It iterates over sequences for us, freeing us from having 73 to deal directly with iterators or write predicates. 74 </p> 75<p> 76 <code class="literal">BOOST_FOREACH</code> is designed for ease-of-use and efficiency. 77 It does no dynamic allocations, makes no virtual function calls or calls through 78 function pointers, and makes no calls that are not transparent to the compiler's 79 optimizer. This results in near-optimal code generation; the performance of 80 <code class="literal">BOOST_FOREACH</code> is usually within a few percent of the equivalent 81 hand-coded loop. And although <code class="literal">BOOST_FOREACH</code> is a macro, 82 it is a remarkably well-behaved one. It evaluates its arguments exactly once, 83 leading to no nasty surprises. 84 </p> 85<h3> 86<a name="foreach.introduction.h1"></a> 87 <span class="phrase"><a name="foreach.introduction.hello__world_"></a></span><a class="link" href="foreach.html#foreach.introduction.hello__world_">Hello, 88 world!</a> 89 </h3> 90<p> 91 Below is a sample program that uses <code class="literal">BOOST_FOREACH</code> to loop 92 over the contents of a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>. 93 </p> 94<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> 95<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 96<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">foreach</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 97 98<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 99<span class="special">{</span> 100 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">hello</span><span class="special">(</span> <span class="string">"Hello, world!"</span> <span class="special">);</span> 101 102 <span class="identifier">BOOST_FOREACH</span><span class="special">(</span> <span class="keyword">char</span> <span class="identifier">ch</span><span class="special">,</span> <span class="identifier">hello</span> <span class="special">)</span> 103 <span class="special">{</span> 104 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">ch</span><span class="special">;</span> 105 <span class="special">}</span> 106 107 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 108<span class="special">}</span> 109</pre> 110<p> 111 This program outputs the following: 112 </p> 113<pre class="programlisting">Hello, world! 114</pre> 115<h3> 116<a name="foreach.introduction.h2"></a> 117 <span class="phrase"><a name="foreach.introduction.supported_sequence_types"></a></span><a class="link" href="foreach.html#foreach.introduction.supported_sequence_types">Supported 118 Sequence Types</a> 119 </h3> 120<p> 121 <code class="literal">BOOST_FOREACH</code> iterates over sequences. But what qualifies 122 as a sequence, exactly? Since <code class="literal">BOOST_FOREACH</code> is built on 123 top of <a href="../../libs/range/index.html" target="_top">Boost.Range</a>, it automatically 124 supports those types which <a href="../../libs/range/index.html" target="_top">Boost.Range</a> 125 recognizes as sequences. Specifically, <code class="literal">BOOST_FOREACH</code> works 126 with types that satisfy the <a href="../../libs/range/doc/html/range/concepts/single_pass_range.html" target="_top">Single 127 Pass Range Concept</a>. For example, we can use <code class="literal">BOOST_FOREACH</code> 128 with: 129 </p> 130<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 131<li class="listitem"> 132 STL containers 133 </li> 134<li class="listitem"> 135 arrays 136 </li> 137<li class="listitem"> 138 Null-terminated strings (<code class="computeroutput"><span class="keyword">char</span></code> 139 and <code class="computeroutput"><span class="keyword">wchar_t</span></code>) 140 </li> 141<li class="listitem"> 142 std::pair of iterators 143 </li> 144</ul></div> 145<div class="note"><table border="0" summary="Note"> 146<tr> 147<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td> 148<th align="left">Note</th> 149</tr> 150<tr><td align="left" valign="top"><p> 151 The support for STL containers is very general; anything that looks like 152 an STL container counts. If it has nested <code class="computeroutput"><span class="identifier">iterator</span></code> 153 and <code class="computeroutput"><span class="identifier">const_iterator</span></code> types 154 and <code class="computeroutput"><span class="identifier">begin</span><span class="special">()</span></code> 155 and <code class="computeroutput"><span class="identifier">end</span><span class="special">()</span></code> 156 member functions, <code class="literal">BOOST_FOREACH</code> will automatically know 157 how to iterate over it. It is in this way that <a href="../../libs/range/doc/html/range/reference/utilities/iterator_range.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">iterator_range</span><span class="special"><></span></code></a> 158 and <a href="../../libs/range/doc/html/range/reference/utilities/sub_range.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sub_range</span><span class="special"><></span></code></a> 159 work with <code class="literal">BOOST_FOREACH</code>. 160 </p></td></tr> 161</table></div> 162<p> 163 See the section on <a class="link" href="foreach/extensibility.html" title="Extensibility">Extensibility</a> 164 to find out how to make <code class="literal">BOOST_FOREACH</code> work with other types. 165 </p> 166<h3> 167<a name="foreach.introduction.h3"></a> 168 <span class="phrase"><a name="foreach.introduction.examples"></a></span><a class="link" href="foreach.html#foreach.introduction.examples">Examples</a> 169 </h3> 170<p> 171 Below are some examples that demonstrate all the different ways we can use 172 <code class="literal">BOOST_FOREACH</code>. 173 </p> 174<p> 175 Iterate over an STL container: 176 </p> 177<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">list_int</span><span class="special">(</span> <span class="comment">/*...*/</span> <span class="special">);</span> 178<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">list_int</span> <span class="special">)</span> 179<span class="special">{</span> 180 <span class="comment">// do something with i</span> 181<span class="special">}</span> 182</pre> 183<p> 184 Iterate over an array, with covariance (i.e., the type of the iteration variable 185 is not exactly the same as the element type of the container): 186 </p> 187<pre class="programlisting"><span class="keyword">short</span> <span class="identifier">array_short</span><span class="special">[]</span> <span class="special">=</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> 188<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">array_short</span> <span class="special">)</span> 189<span class="special">{</span> 190 <span class="comment">// The short was implicitly converted to an int</span> 191<span class="special">}</span> 192</pre> 193<p> 194 Predeclare the loop variable, and use <code class="computeroutput"><span class="keyword">break</span></code>, 195 <code class="computeroutput"><span class="keyword">continue</span></code>, and <code class="computeroutput"><span class="keyword">return</span></code> 196 in the loop body: 197 </p> 198<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">deque</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">deque_int</span><span class="special">(</span> <span class="comment">/*...*/</span> <span class="special">);</span> 199<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 200<span class="identifier">BOOST_FOREACH</span><span class="special">(</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">deque_int</span> <span class="special">)</span> 201<span class="special">{</span> 202 <span class="keyword">if</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">==</span> <span class="number">0</span> <span class="special">)</span> <span class="keyword">return</span><span class="special">;</span> 203 <span class="keyword">if</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="keyword">continue</span><span class="special">;</span> 204 <span class="keyword">if</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">==</span> <span class="number">2</span> <span class="special">)</span> <span class="keyword">break</span><span class="special">;</span> 205<span class="special">}</span> 206</pre> 207<p> 208 Iterate over a sequence by reference, and modify the underlying sequence: 209 </p> 210<pre class="programlisting"><span class="keyword">short</span> <span class="identifier">array_short</span><span class="special">[]</span> <span class="special">=</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> 211<span class="identifier">BOOST_FOREACH</span><span class="special">(</span> <span class="keyword">short</span> <span class="special">&</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">array_short</span> <span class="special">)</span> 212<span class="special">{</span> 213 <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> 214<span class="special">}</span> 215<span class="comment">// array_short contains {2,3,4} here</span> 216</pre> 217<p> 218 Iterate over a vector of vectors with nested <code class="literal">BOOST_FOREACH</code> 219 loops. In this example, notice that braces around the loop body are not necessary: 220 </p> 221<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</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="special">></span> <span class="identifier">matrix_int</span><span class="special">;</span> 222<span class="identifier">BOOST_FOREACH</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="special">&</span> <span class="identifier">row</span><span class="special">,</span> <span class="identifier">matrix_int</span> <span class="special">)</span> 223 <span class="identifier">BOOST_FOREACH</span><span class="special">(</span> <span class="keyword">int</span> <span class="special">&</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">row</span> <span class="special">)</span> 224 <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> 225</pre> 226<p> 227 Iterate over an expression that returns a sequence by value (i.e. an rvalue): 228 </p> 229<pre class="programlisting"><span class="keyword">extern</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span> <span class="identifier">get_vector_float</span><span class="special">();</span> 230<span class="identifier">BOOST_FOREACH</span><span class="special">(</span> <span class="keyword">float</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">get_vector_float</span><span class="special">()</span> <span class="special">)</span> 231<span class="special">{</span> 232 <span class="comment">// Note: get_vector_float() will be called exactly once</span> 233<span class="special">}</span> 234</pre> 235<p> 236 Iterate in reverse: 237 </p> 238<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">list_int</span><span class="special">(</span> <span class="comment">/*...*/</span> <span class="special">);</span> 239<span class="identifier">BOOST_REVERSE_FOREACH</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">list_int</span> <span class="special">)</span> 240<span class="special">{</span> 241 <span class="comment">// do something with i</span> 242<span class="special">}</span> 243</pre> 244<p> 245 Iterating over rvalues doesn't work on some older compilers. Check the <a class="link" href="foreach/portability.html" title="Portability">Portability</a> section to see whether your 246 compiler supports this. 247 </p> 248<h3> 249<a name="foreach.introduction.h4"></a> 250 <span class="phrase"><a name="foreach.introduction.making__literal_boost_foreach__literal__prettier"></a></span><a class="link" href="foreach.html#foreach.introduction.making__literal_boost_foreach__literal__prettier">Making 251 <code class="literal">BOOST_FOREACH</code> Prettier</a> 252 </h3> 253<p> 254 People have complained about the name <code class="literal">BOOST_FOREACH</code>. It's 255 too long. <code class="computeroutput"><span class="identifier">ALL</span> <span class="identifier">CAPS</span></code> 256 can get tiresome to look at. That may be true, but <code class="literal">BOOST_FOREACH</code> 257 is merely following the <a href="http://www.boost.org/more/lib_guide.htm" target="_top">Boost 258 Naming Convention</a>. That doesn't mean you're stuck with it, though. 259 If you would like to use a different identifier (<code class="computeroutput"><span class="identifier">foreach_</span></code>, 260 perhaps), you can simply do: 261 </p> 262<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">foreach_</span> <span class="identifier">BOOST_FOREACH</span> 263<span class="preprocessor">#define</span> <span class="identifier">foreach_r_</span> <span class="identifier">BOOST_REVERSE_FOREACH</span> 264</pre> 265<p> 266 Only do this if you are sure that the identifier you choose will not cause 267 name conflicts in your code. 268 </p> 269<div class="note"><table border="0" summary="Note"> 270<tr> 271<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td> 272<th align="left">Note</th> 273</tr> 274<tr><td align="left" valign="top"><p> 275 Do not use <code class="computeroutput"><span class="preprocessor">#define</span> <span class="identifier">foreach_</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span><span class="identifier">y</span><span class="special">)</span> <span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span><span class="identifier">y</span><span class="special">)</span></code>. This 276 can be problematic if the arguments are macros themselves. This would result 277 in an additional expansion of these macros. Instead, use the form shown above. 278 </p></td></tr> 279</table></div> 280<p> 281 Lastly, a word of warning. Lots of folks use a <code class="computeroutput"><span class="identifier">foreach</span></code> 282 macro as a short form for <code class="computeroutput"><span class="identifier">BOOST_FOREACH</span></code>. 283 I discourage this. It leads to name conflicts within the <code class="computeroutput"><span class="identifier">BOOST_FOREACH</span></code> 284 macro itself, where <code class="computeroutput"><span class="identifier">foreach</span></code> 285 is the name of a namespace. Besides, <code class="computeroutput"><span class="identifier">foreach</span></code> 286 is a common-enough identifier; even <a href="http://qt.digia.com/" target="_top">Qt</a> 287 defines it as a macro. If you insist on using <code class="computeroutput"><span class="identifier">foreach</span></code>, 288 you might try something like this: 289 </p> 290<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">foreach</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 291 292<span class="keyword">namespace</span> <span class="identifier">boost</span> 293<span class="special">{</span> 294 <span class="comment">// Suggested work-around for https://svn.boost.org/trac/boost/ticket/6131</span> 295 <span class="keyword">namespace</span> <span class="identifier">BOOST_FOREACH</span> <span class="special">=</span> <span class="identifier">foreach</span><span class="special">;</span> 296<span class="special">}</span> 297 298<span class="preprocessor">#define</span> <span class="identifier">foreach</span> <span class="identifier">BOOST_FOREACH</span> 299</pre> 300<p> 301 This will work around <span class="emphasis"><em>some</em></span> of the problems you're likely 302 to encounter, but not all. Prefer using a different identifier. 303 </p> 304</div> 305</div> 306<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 307<td align="left"><p><small>Last revised: August 11, 2020 at 15:03:46 GMT</small></p></td> 308<td align="right"><div class="copyright-footer"></div></td> 309</tr></table> 310<hr> 311<div class="spirit-nav"> 312<a accesskey="p" href="boost_dll/acknowledgements.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="foreach/extensibility.html"><img src="../../doc/src/images/next.png" alt="Next"></a> 313</div> 314</body> 315</html> 316
