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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="foreach.extensibility"></a><a class="link" href="extensibility.html" title="Extensibility">Extensibility</a>
</h2></div></div></div>
<p>
If we want to use <code class="literal">BOOST_FOREACH</code> to iterate over some new
collection type, we must "teach" <code class="literal">BOOST_FOREACH</code>
how to interact with our type. Since <code class="literal">BOOST_FOREACH</code> is built
on top of <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>, we
must extend <a href="../../../libs/range/index.html" target="_top">Boost.Range</a> in
order to extend <code class="literal">BOOST_FOREACH</code>. The section <a href="../../../libs/range/doc/html/range/reference/extending.html" target="_top">Extending
Boost.Range</a> explores this topic in detail.
</p>
<p>
Below is an example for extending <code class="literal">BOOST_FOREACH</code> to iterate
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>.
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">my</span>
<span class="special">{</span>
<span class="comment">// sub_string: part of a string, as delimited by a pair
</span> <span class="comment">// of iterators
</span> <span class="keyword">struct</span> <span class="identifier">sub_string</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="identifier">begin</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="identifier">end</span><span class="special">;</span>
<span class="comment">/* ... implementation ... */</span>
<span class="special">};</span>
<span class="comment">// Add overloads of range_begin() and range_end() in the
</span> <span class="comment">// same namespace as sub_string, to be found by Argument-Dependent Lookup.
</span>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// Also add overloads for const sub_strings. Note we use the conversion
</span> <span class="comment">// from string::iterator to string::const_iterator here.
</span>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">begin</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span>
<span class="special">{</span>
<span class="comment">// specialize range_mutable_iterator and range_const_iterator in namespace boost
</span> <span class="keyword">template</span><span class="special"><></span>
<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>
<span class="special">{</span>
<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>
<span class="special">};</span>
<span class="keyword">template</span><span class="special"><></span>
<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>
<span class="special">{</span>
<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>
<span class="special">};</span>
<span class="special">}</span>
</pre>
<p>
Now that we have taught <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>
(and hence <code class="literal">BOOST_FOREACH</code>) about our type, we can now use
<code class="literal">BOOST_FOREACH</code> to iterate over our sub_string type.
</p>
<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>
<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>
<span class="special">{</span>
<span class="comment">// Woo-hoo!
</span><span class="special">}</span>
</pre>
<p>
There are some portability issues we should be aware of when extending <code class="literal">BOOST_FOREACH</code>.
Be sure to check out the <a class="link" href="portability.html" title="Portability">Portability</a>
section. In particular, if your compiler does not support Argument-Dependent
Lookup, the <a href="../../../libs/range/doc/html/range/portability.html" target="_top">Boost.Range
Portability</a> section offers some suggested work-arounds.
</p>
<a name="foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types"></a><h3>
<a name="id1260705"></a>
<a class="link" href="extensibility.html#foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types">Making
<code class="literal">BOOST_FOREACH</code> Work with Non-Copyable Sequence Types</a>
</h3>
<p>
For sequence types that are non-copyable, we will need to tell <code class="literal">BOOST_FOREACH</code>
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>,
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:
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">noncopy_vector</span>
<span class="special">{</span>
<span class="comment">// ...
</span><span class="keyword">private</span><span class="special">:</span>
<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><span class="special">};</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">foreach</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><></span>
<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>
<span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{</span>
<span class="special">};</span>
<span class="special">}}</span>
</pre>
<p>
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>
function. Doing it this way has the advantage of being portable to older compilers.
</p>
<pre class="programlisting"><span class="comment">// At global scope...
</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>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
Even though we have to tell <code class="literal">BOOST_FOREACH</code> that our type
is non-copyable, that doesn't mean that <code class="literal">BOOST_FOREACH</code>
always makes a copy of our sequence type. Obviously, doing so would be expensive
and even wrong in some cases. <code class="literal">BOOST_FOREACH</code> is quite smart
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,
which is on a branch that might never get taken.
</p></td></tr>
</table></div>
<a name="foreach.extensibility.optimizing__literal_boost_foreach__literal__for_lightweight_proxy_sequence_types"></a><h3>
<a name="id1261166"></a>
<a class="link" href="extensibility.html#foreach.extensibility.optimizing__literal_boost_foreach__literal__for_lightweight_proxy_sequence_types">Optimizing
<code class="literal">BOOST_FOREACH</code> for Lightweight Proxy Sequence Types</a>
</h3>
<p>
On some compilers, <code class="literal">BOOST_FOREACH</code> must occasionally take
a slightly slower code path to guarantee correct handling of sequences stored
in temporary objects. It asks itself, "Should I make a copy of this object?"
and later, "Did I make a copy or not?" For some types of sequences,
this is overkill. Consider a sequence which is a simple pair of iterators.
Jumping through hoops of fire to avoid copying it doesn't make sense because
copying it is so cheap.
</p>
<p>
A pair of iterators is an example of a lightweight proxy. It does not store
the values of the sequence; rather, it stores iterators to them. This means
that iterating over a copy of the proxy object will give the same results as
using the object itself. For such types, <code class="literal">BOOST_FOREACH</code> provides
a hook that lets us tell it not to worry about the expense of making a copy.
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:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">sub_string</span>
<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>
<span class="special">{</span>
<span class="comment">// ...
</span><span class="special">};</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">foreach</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><></span>
<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>
<span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{</span>
<span class="special">};</span>
<span class="special">}}</span>
</pre>
<p>
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>
function, as follows:
</p>
<pre class="programlisting"><span class="comment">// At global scope...
</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>
<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>
<span class="special">{</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
This method is portable to older compilers.
</p>
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<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>
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