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| <div class="section"> |
| <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="id1260690"></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="id1261152"></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> |
| </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> |
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