ceph/src/boost/libs/spirit/classic/phoenix/doc/quick_start.html

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  15       <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Quick start</b></font>
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  28 <p>
  29 To get a first glimpse on what the Phoenix framework offers, let us start with an example. We want to find the first odd number in an STL container.</p>
  30 <p>
  31 1)  Normally we use a functor or a function pointer and pass that in to STL's find_if generic function (sample1.cpp):</p>
  32 <p>
  33 Write a function:</p>
  34 <code><pre>
  35     <span class=keyword>bool
  36     </span><span class=identifier>is_odd</span><span class=special>(</span><span class=keyword>int </span><span class=identifier>arg1</span><span class=special>)
  37     {
  38         </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>;
  39     }
  40 </span></pre></code>
  41 <p>
  42 Pass a pointer to the function to STL's find_if generic function:</p>
  43 <code><pre>
  44     <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>(), &amp;</span><span class=identifier>is_odd</span><span class=special>)
  45 </span></pre></code>
  46 <p>
  47 2)  Using Phoenix, the same can be achieved directly with a one- liner (sample2.cpp):</p>
  48 <code><pre>
  49     <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>arg1 </span><span class=special>% </span><span class=number>2 </span><span class=special>== </span><span class=number>1</span><span class=special>)
  50 </span></pre></code>
  51 <p>
  52 The expression &quot;arg1 % 2 == 1&quot; automagically creates a functor with the expected behavior. In FP, this unnamed function is called a lambda function. Unlike 1, the function pointer version, which is monomorphic (expects and works only with a fixed type int argument), the Phoenix version is completely polymorphic and works with any container (of ints, of doubles, of complex, etc.) as long as its elements can handle the &quot;arg1 % 2 == 1&quot; expression.</p>
  53 <p>
  54 3) Write a polymorphic functor using Phoenix (sample3.cpp)</p>
  55 <code><pre>
  56     <span class=keyword>struct </span><span class=identifier>is_odd_ </span><span class=special>{
  57
  58         </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ArgT</span><span class=special>&gt;
  59         </span><span class=keyword>struct </span><span class=identifier>result </span><span class=special>{ </span><span class=keyword>typedef </span><span class=keyword>bool </span><span class=identifier>type</span><span class=special>; };
  60
  61         </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ArgT</span><span class=special>&gt;
  62         </span><span class=keyword>bool </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>ArgT </span><span class=identifier>arg1</span><span class=special>) </span><span class=keyword>const
  63         </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>; }
  64     };
  65
  66     </span><span class=identifier>function</span><span class=special>&lt;</span><span class=identifier>is_odd_</span><span class=special>&gt; </span><span class=identifier>is_odd</span><span class=special>;
  67 </span></pre></code>
  68 <p>
  69 Call the lazy is_odd function:</p>
  70 <code><pre>
  71     <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>))
  72 </span></pre></code>
  73 <p>
  74 is_odd_ is the actual functor. It has been proxied in function&lt;is_odd_&gt; by is_odd (note no trailing underscore) which makes it a lazy function. is_odd_::operator() is the main function body. is_odd_::result is a type computer that answers the question &quot;What should be our return type given an argument of type ArgT?&quot;.</p>
  75 <p>
  76 Like 2, and unlike 1, function pointers or plain C++ functors, is_odd is a true lazy, polymorphic functor (rank-2 polymorphic functoid, in <a href="http://www.cc.gatech.edu/~yannis/fc++/">
  77 FC++</a> jargon). The Phoenix functor version is fully polymorphic and works with any container (of ints, of doubles, of complex, etc.) as long as its elements can handle the &quot;arg1 % 2 == 1&quot; expression. However, unlike 2, this is more efficient and has less overhead especially when dealing with much more complex functions.</p>
  78 <p>
  79 This is just the tip of the iceberg. There are more nifty things you can do with the framework. There are quite interesting concepts such as rank-2 polymorphic lazy functions, lazy statements, binders etc; enough to whet the appetite of anyone wishing to squeeze more power from C++.</p>
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  89 <p class="copyright">Copyright &copy; 2001-2002 Joel de Guzman<br>
  90   <br>
  91 <font size="2">Use, modification and distribution is subject to the Boost Software
  92     License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
  93     http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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