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>Extensibility</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="../foreach.html" title="Chapter 15. Boost.Foreach"> 10<link rel="prev" href="../foreach.html" title="Chapter 15. Boost.Foreach"> 11<link rel="next" href="portability.html" title="Portability"> 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="../foreach.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../foreach.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="portability.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="foreach.extensibility"></a><a class="link" href="extensibility.html" title="Extensibility">Extensibility</a> 29</h2></div></div></div> 30<p> 31 If we want to use <code class="literal">BOOST_FOREACH</code> to iterate over some new 32 collection type, we must "teach" <code class="literal">BOOST_FOREACH</code> 33 how to interact with our type. Since <code class="literal">BOOST_FOREACH</code> is built 34 on top of <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>, we 35 must extend <a href="../../../libs/range/index.html" target="_top">Boost.Range</a> in 36 order to extend <code class="literal">BOOST_FOREACH</code>. The section <a href="../../../libs/range/doc/html/range/reference/extending.html" target="_top">Extending 37 Boost.Range</a> explores this topic in detail. 38 </p> 39<p> 40 Below is an example for extending <code class="literal">BOOST_FOREACH</code> to iterate 41 over a sub-string type, which contains two iterators into a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>. 42 </p> 43<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">my</span> 44<span class="special">{</span> 45 <span class="comment">// sub_string: part of a string, as delimited by a pair</span> 46 <span class="comment">// of iterators</span> 47 <span class="keyword">struct</span> <span class="identifier">sub_string</span> 48 <span class="special">{</span> 49 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">begin</span><span class="special">;</span> 50 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">end</span><span class="special">;</span> 51 52 <span class="comment">/* ... implementation ... */</span> 53 <span class="special">};</span> 54 55 <span class="comment">// Add overloads of range_begin() and range_end() in the</span> 56 <span class="comment">// same namespace as sub_string, to be found by Argument-Dependent Lookup.</span> 57 58 <span class="keyword">inline</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">range_begin</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> 59 <span class="special">{</span> 60 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span> 61 <span class="special">}</span> 62 63 <span class="keyword">inline</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">range_end</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> 64 <span class="special">{</span> 65 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span> 66 <span class="special">}</span> 67 68 <span class="comment">// Also add overloads for const sub_strings. Note we use the conversion</span> 69 <span class="comment">// from string::iterator to string::const_iterator here.</span> 70 71 <span class="keyword">inline</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">range_begin</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> 72 <span class="special">{</span> 73 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span> 74 <span class="special">}</span> 75 76 <span class="keyword">inline</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">range_end</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> 77 <span class="special">{</span> 78 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span> 79 <span class="special">}</span> 80<span class="special">}</span> 81 82<span class="keyword">namespace</span> <span class="identifier">boost</span> 83<span class="special">{</span> 84 <span class="comment">// specialize range_mutable_iterator and range_const_iterator in namespace boost</span> 85 <span class="keyword">template</span><span class="special"><></span> 86 <span class="keyword">struct</span> <span class="identifier">range_mutable_iterator</span><span class="special"><</span> <span class="identifier">my</span><span class="special">::</span><span class="identifier">sub_string</span> <span class="special">></span> 87 <span class="special">{</span> 88 <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">type</span><span class="special">;</span> 89 <span class="special">};</span> 90 91 <span class="keyword">template</span><span class="special"><></span> 92 <span class="keyword">struct</span> <span class="identifier">range_const_iterator</span><span class="special"><</span> <span class="identifier">my</span><span class="special">::</span><span class="identifier">sub_string</span> <span class="special">></span> 93 <span class="special">{</span> 94 <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">type</span><span class="special">;</span> 95 <span class="special">};</span> 96<span class="special">}</span> 97</pre> 98<p> 99 Now that we have taught <a href="../../../libs/range/index.html" target="_top">Boost.Range</a> 100 (and hence <code class="literal">BOOST_FOREACH</code>) about our type, we can now use 101 <code class="literal">BOOST_FOREACH</code> to iterate over our sub_string type. 102 </p> 103<pre class="programlisting"><span class="identifier">my</span><span class="special">::</span><span class="identifier">sub_string</span> <span class="identifier">substr</span><span class="special">;</span> 104<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">substr</span> <span class="special">)</span> 105<span class="special">{</span> 106 <span class="comment">// Woo-hoo!</span> 107<span class="special">}</span> 108</pre> 109<p> 110 There are some portability issues we should be aware of when extending <code class="literal">BOOST_FOREACH</code>. 111 Be sure to check out the <a class="link" href="portability.html" title="Portability">Portability</a> 112 section. In particular, if your compiler does not support Argument-Dependent 113 Lookup, the <a href="../../../libs/range/doc/html/range/portability.html" target="_top">Boost.Range 114 Portability</a> section offers some suggested work-arounds. 115 </p> 116<h3> 117<a name="foreach.extensibility.h0"></a> 118 <span class="phrase"><a name="foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types"></a></span><a class="link" href="extensibility.html#foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types">Making 119 <code class="literal">BOOST_FOREACH</code> Work with Non-Copyable Sequence Types</a> 120 </h3> 121<p> 122 For sequence types that are non-copyable, we will need to tell <code class="literal">BOOST_FOREACH</code> 123 to not try to make copies. If our type inherits from <a href="../../../libs/utility/utility.htm#Class_noncopyable" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code></a>, 124 no further action is required. If not, we must specialize the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">foreach</span><span class="special">::</span><span class="identifier">is_noncopyable</span><span class="special"><></span></code> template, as follows: 125 </p> 126<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">noncopy_vector</span> 127<span class="special">{</span> 128 <span class="comment">// ...</span> 129<span class="keyword">private</span><span class="special">:</span> 130 <span class="identifier">noncopy_vector</span><span class="special">(</span> <span class="identifier">noncopy_vector</span> <span class="keyword">const</span> <span class="special">&</span> <span class="special">);</span> <span class="comment">// non-copyable!</span> 131<span class="special">};</span> 132 133<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">foreach</span> 134<span class="special">{</span> 135 <span class="keyword">template</span><span class="special"><></span> 136 <span class="keyword">struct</span> <span class="identifier">is_noncopyable</span><span class="special"><</span> <span class="identifier">noncopy_vector</span> <span class="special">></span> 137 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 138 <span class="special">{</span> 139 <span class="special">};</span> 140<span class="special">}}</span> 141</pre> 142<p> 143 Another way to achieve the same effect is to override the global <code class="computeroutput"><span class="identifier">boost_foreach_is_noncopyable</span><span class="special">()</span></code> 144 function. Doing it this way has the advantage of being portable to older compilers. 145 </p> 146<pre class="programlisting"><span class="comment">// At global scope...</span> 147<span class="keyword">inline</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> <span class="special">*</span> 148<span class="identifier">boost_foreach_is_noncopyable</span><span class="special">(</span> <span class="identifier">noncopy_vector</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">tag</span> <span class="special">)</span> 149<span class="special">{</span> 150 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 151<span class="special">}</span> 152</pre> 153<div class="tip"><table border="0" summary="Tip"> 154<tr> 155<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> 156<th align="left">Tip</th> 157</tr> 158<tr><td align="left" valign="top"><p> 159 Even though we have to tell <code class="literal">BOOST_FOREACH</code> that our type 160 is non-copyable, that doesn't mean that <code class="literal">BOOST_FOREACH</code> 161 always makes a copy of our sequence type. Obviously, doing so would be expensive 162 and even wrong in some cases. <code class="literal">BOOST_FOREACH</code> is quite smart 163 about when to make a copy and when not to. The <code class="computeroutput"><span class="identifier">is_noncopyable</span><span class="special"><></span></code> trait is needed to elide the copy, 164 which is on a branch that might never get taken. 165 </p></td></tr> 166</table></div> 167<h3> 168<a name="foreach.extensibility.h1"></a> 169 <span class="phrase"><a name="foreach.extensibility.optimizing__literal_boost_foreach__literal__for_lightweight_proxy_sequence_types"></a></span><a class="link" href="extensibility.html#foreach.extensibility.optimizing__literal_boost_foreach__literal__for_lightweight_proxy_sequence_types">Optimizing 170 <code class="literal">BOOST_FOREACH</code> for Lightweight Proxy Sequence Types</a> 171 </h3> 172<p> 173 On some compilers, <code class="literal">BOOST_FOREACH</code> must occasionally take 174 a slightly slower code path to guarantee correct handling of sequences stored 175 in temporary objects. It asks itself, "Should I make a copy of this object?" 176 and later, "Did I make a copy or not?" For some types of sequences, 177 this is overkill. Consider a sequence which is a simple pair of iterators. 178 Jumping through hoops of fire to avoid copying it doesn't make sense because 179 copying it is so cheap. 180 </p> 181<p> 182 A pair of iterators is an example of a lightweight proxy. It does not store 183 the values of the sequence; rather, it stores iterators to them. This means 184 that iterating over a copy of the proxy object will give the same results as 185 using the object itself. For such types, <code class="literal">BOOST_FOREACH</code> provides 186 a hook that lets us tell it not to worry about the expense of making a copy. 187 This can result in slightly faster loop execution. Simply specialize the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">foreach</span><span class="special">::</span><span class="identifier">is_lightweight_proxy</span><span class="special"><></span></code> trait, as follows: 188 </p> 189<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">sub_string</span> 190 <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">iterator_range</span><span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="special">></span> 191<span class="special">{</span> 192 <span class="comment">// ...</span> 193<span class="special">};</span> 194 195<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">foreach</span> 196<span class="special">{</span> 197 <span class="keyword">template</span><span class="special"><></span> 198 <span class="keyword">struct</span> <span class="identifier">is_lightweight_proxy</span><span class="special"><</span> <span class="identifier">sub_string</span> <span class="special">></span> 199 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 200 <span class="special">{</span> 201 <span class="special">};</span> 202<span class="special">}}</span> 203</pre> 204<p> 205 Alternately, we could achieve the same effect by overriding the global <code class="computeroutput"><span class="identifier">boost_foreach_is_lightweight_proxy</span><span class="special">()</span></code> 206 function, as follows: 207 </p> 208<pre class="programlisting"><span class="comment">// At global scope...</span> 209<span class="keyword">inline</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> <span class="special">*</span> 210<span class="identifier">boost_foreach_is_lightweight_proxy</span><span class="special">(</span> <span class="identifier">sub_string</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">tag</span> <span class="special">)</span> 211<span class="special">{</span> 212 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 213<span class="special">}</span> 214</pre> 215<p> 216 This method is portable to older compilers. 217 </p> 218</div> 219<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 220<td align="left"></td> 221<td align="right"><div class="copyright-footer">Copyright © 2004 Eric Niebler<p> 222 Distributed under the Boost Software License, Version 1.0. (See accompanying 223 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>) 224 </p> 225</div></td> 226</tr></table> 227<hr> 228<div class="spirit-nav"> 229<a accesskey="p" href="../foreach.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../foreach.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="portability.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 230</div> 231</body> 232</html> 233
