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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.starter_kit.composites.lazy_functions"></a><a class="link" href="lazy_functions.html" title="Lazy Functions">Lazy |
| Functions</a> |
| </h4></div></div></div> |
| <p> |
| As you write more lambda functions, you'll notice certain patterns that |
| you wish to refactor as reusable functions. When you reach that point, |
| you'll wish that ordinary functions can co-exist with phoenix functions. |
| Unfortunately, the <span class="emphasis"><em>immediate</em></span> nature of plain C++ functions |
| make them incompatible. |
| </p> |
| <p> |
| Lazy functions are your friends. The library provides a facility to make |
| lazy functions. The code below is a rewrite of the <code class="computeroutput"><span class="identifier">is_odd</span></code> |
| function using the facility: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">is_odd_impl</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">bool</span> <span class="identifier">result_type</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> |
| <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="identifier">arg1</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span> <span class="identifier">is_odd</span><span class="special">;</span> |
| </pre> |
| <a name="phoenix.starter_kit.composites.lazy_functions.things_to_note_"></a><h6> |
| <a name="id683362"></a> |
| <a class="link" href="lazy_functions.html#phoenix.starter_kit.composites.lazy_functions.things_to_note_">Things |
| to note:</a> |
| </h6> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Result type deduction is implemented with the help of the result_of |
| protocol. For more information see <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result |
| Of</a> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">is_odd_impl</span></code> implements |
| the function. |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">is_odd</span></code>, an instance |
| of <code class="computeroutput"><span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span></code>, |
| is the lazy function. |
| </li> |
| </ul></div> |
| <p> |
| Now, <code class="computeroutput"><span class="identifier">is_odd</span></code> is a truly |
| lazy function that we can use in conjunction with the rest of phoenix. |
| Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">is_odd</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">));</span> |
| </pre> |
| <p> |
| (See <a href="../../../../../example/function.cpp" target="_top">function.cpp</a>) |
| </p> |
| <a name="phoenix.starter_kit.composites.lazy_functions.predefined_lazy_functions"></a><h6> |
| <a name="id683541"></a> |
| <a class="link" href="lazy_functions.html#phoenix.starter_kit.composites.lazy_functions.predefined_lazy_functions">Predefined |
| Lazy Functions</a> |
| </h6> |
| <p> |
| The library is chock full of STL savvy, predefined lazy functions covering |
| the whole of the STL containers, iterators and algorithms. For example, |
| there are lazy versions of container related operations such as assign, |
| at, back, begin, pop_back, pop_front, push_back, push_front, etc. (See |
| <a class="link" href="../../modules/stl.html" title="STL">STL</a>). |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<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>) |
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