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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="foreach.extensibility"></a>Extensibility</h2></div></div></div>
28<p>
29      If we want to use <code class="literal">BOOST_FOREACH</code> to iterate over some new
30      collection type, we must "teach" <code class="literal">BOOST_FOREACH</code>
31      how to interact with our type. Since <code class="literal">BOOST_FOREACH</code> is built
32      on top of <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>, we
33      must extend <a href="../../../libs/range/index.html" target="_top">Boost.Range</a> in
34      order to extend <code class="literal">BOOST_FOREACH</code>. The section <a href="../../../libs/range/doc/boost_range.html#minimal_interface" target="_top">Extending
35      Boost.Range</a> explores this topic in detail.
36    </p>
37<p>
38      Below is an example for extending <code class="literal">BOOST_FOREACH</code> to iterate
39      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>.
40    </p>
41<pre class="programlisting">
42<span class="keyword">namespace</span> <span class="identifier">my</span>
43<span class="special">{</span>
44    <span class="comment">// sub_string: part of a string, as delimited by a pair
45</span>    <span class="comment">// of iterators
46</span>    <span class="keyword">struct</span> <span class="identifier">sub_string</span>
47    <span class="special">{</span>
48        <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>
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">end</span><span class="special">;</span>
50       
51        <span class="comment">/* ... implementation ... */</span>
52    <span class="special">};</span>
53
54    <span class="comment">// Add overloads of boost_range_begin() and boost_range_end() in the
55</span>    <span class="comment">// same namespace as sub_string, to be found by Argument-Dependent Lookup.
56</span>
57    <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">boost_range_begin</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span>
58    <span class="special">{</span>
59        <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span>
60    <span class="special">}</span>
61
62    <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">boost_range_end</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span>
63    <span class="special">{</span>
64        <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span>
65    <span class="special">}</span>
66
67    <span class="comment">// Also add overloads for const sub_strings. Note we use the conversion
68</span>    <span class="comment">// from string::iterator to string::const_iterator here.
69</span>
70    <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">boost_range_begin</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span>
71    <span class="special">{</span>
72        <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span>
73    <span class="special">}</span>
74
75    <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">boost_range_end</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span>
76    <span class="special">{</span>
77        <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span>
78    <span class="special">}</span>
79<span class="special">}</span>
80
81<span class="keyword">namespace</span> <span class="identifier">boost</span>
82<span class="special">{</span>
83    <span class="comment">// specialize rannge_iterator and range_const_iterator in namespace boost
84</span>    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
85    <span class="keyword">struct</span> <span class="identifier">range_iterator</span><span class="special">&lt;</span> <span class="identifier">my</span><span class="special">::</span><span class="identifier">sub_string</span> <span class="special">&gt;</span>
86    <span class="special">{</span>
87        <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>
88    <span class="special">};</span>
89
90    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
91    <span class="keyword">struct</span> <span class="identifier">range_const_iterator</span><span class="special">&lt;</span> <span class="identifier">my</span><span class="special">::</span><span class="identifier">sub_string</span> <span class="special">&gt;</span>
92    <span class="special">{</span>
93        <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>
94    <span class="special">};</span>
95<span class="special">}</span>
96</pre>
97<p>
98      Now that we have taught <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>
99      (and hence <code class="literal">BOOST_FOREACH</code>) about our type, we can now use
100      <code class="literal">BOOST_FOREACH</code> to iterate over our sub_string type.
101    </p>
102<pre class="programlisting">
103<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!
107</span><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 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/portability.html" target="_top">Boost.Range
114      Portability</a> section offers some suggested work-arounds.
115    </p>
116<a name="foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types"></a><h2>
117<a name="id1183459"></a>
118      Making <code class="literal">BOOST_FOREACH</code> Work with Non-Copyable Sequence Types
119    </h2>
120<p>
121      For sequence types that are non-copyable, we will need to tell <code class="literal">BOOST_FOREACH</code>
122      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>,
123      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">&lt;&gt;</span></code> template, as follows:
124    </p>
125<pre class="programlisting">
126<span class="keyword">class</span> <span class="identifier">noncopy_vector</span>
127<span class="special">{</span>
128    <span class="comment">// ...
129</span><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">&amp;</span> <span class="special">);</span> <span class="comment">// non-copyable!
131</span><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">&lt;&gt;</span>
136    <span class="keyword">struct</span> <span class="identifier">is_noncopyable</span><span class="special">&lt;</span> <span class="identifier">noncopy_vector</span> <span class="special">&gt;</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">
147<span class="comment">// At global scope...
148</span><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>
149<span class="identifier">boost_foreach_is_noncopyable</span><span class="special">(</span> <span class="identifier">noncopy_vector</span> <span class="special">*&amp;,</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>
150<span class="special">{</span>
151    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
152<span class="special">}</span>
153</pre>
154<div class="tip"><table border="0" summary="Tip">
155<tr>
156<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../images/tip.png"></td>
157<th align="left">Tip</th>
158</tr>
159<tr><td align="left" valign="top"><p>
160        Even though we have to tell <code class="literal">BOOST_FOREACH</code> that our type
161        is non-copyable, that doesn't mean that <code class="literal">BOOST_FOREACH</code>
162        always makes a copy of our sequence type. Obviously, doing so would be expensive
163        and even wrong in some cases. <code class="literal">BOOST_FOREACH</code> is quite smart
164        about when to make a copy and when not to. The <code class="computeroutput"><span class="identifier">is_noncopyable</span><span class="special">&lt;&gt;</span></code> trait is needed to elide the copy,
165        which is on a branch that might never get taken.
166      </p></td></tr>
167</table></div>
168<a name="foreach.extensibility.optimizing__literal_boost_foreach__literal__for_lightweight_proxy_sequence_types"></a><h2>
169<a name="id1183913"></a>
170      Optimizing <code class="literal">BOOST_FOREACH</code> for Lightweight Proxy Sequence
171      Types
172    </h2>
173<p>
174      On some compilers, <code class="literal">BOOST_FOREACH</code> must occasionally take
175      a slightly slower code path to guarantee correct handling of sequences stored
176      in temporary objects. It asks itself, "Should I make a copy of this object?"
177      and later, "Did I make a copy or not?" For some types of sequences,
178      this is overkill. Consider a sequence which is a simple pair of iterators.
179      Jumping through hoops of fire to avoid copying it doesn't make sense because
180      copying it is so cheap.
181    </p>
182<p>
183      A pair of iterators is an example of a lightweight proxy. It does not store
184      the values of the sequence; rather, it stores iterators to them. This means
185      that iterating over a copy of the proxy object will give the same results as
186      using the object itself. For such types, <code class="literal">BOOST_FOREACH</code> provides
187      a hook that lets us tell it not to worry about the expense of making a copy.
188      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">&lt;&gt;</span></code> trait, as follows:
189    </p>
190<pre class="programlisting">
191<span class="keyword">struct</span> <span class="identifier">sub_string</span>
192  <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">iterator_range</span><span class="special">&lt;</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">&gt;</span>
193<span class="special">{</span>
194    <span class="comment">// ...
195</span><span class="special">};</span>
196
197<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">foreach</span>
198<span class="special">{</span>
199    <span class="keyword">template</span><span class="special">&lt;&gt;</span>
200    <span class="keyword">struct</span> <span class="identifier">is_lightweight_proxy</span><span class="special">&lt;</span> <span class="identifier">sub_string</span> <span class="special">&gt;</span>
201      <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
202    <span class="special">{</span>
203    <span class="special">};</span>
204<span class="special">}}</span>
205</pre>
206<p>
207      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>
208      function, as follows:
209    </p>
210<pre class="programlisting">
211<span class="comment">// At global scope...
212</span><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>
213<span class="identifier">boost_foreach_is_lightweight_proxy</span><span class="special">(</span> <span class="identifier">sub_string</span> <span class="special">*&amp;,</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>
214<span class="special">{</span>
215    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
216<span class="special">}</span>
217</pre>
218<p>
219      This method is portable to older compilers.
220    </p>
221</div>
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224<td align="right"><small>Copyright © 2004 Eric Niebler</small></td>
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