1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Annex: Implementation</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> 8<link rel="up" href="../index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> 9<link rel="prev" href="no_variadic_macros.html" title="Annex: No Variadic Macros"> 10<link rel="next" href="../reference.html" title="Reference"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="no_variadic_macros.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../reference.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="boost_localfunction.implementation"></a><a class="link" href="implementation.html" title="Annex: Implementation">Annex: Implementation</a> 28</h2></div></div></div> 29<p> 30 This section gives an overview of the key programming techniques used to implement 31 this library. 32 </p> 33<div class="note"><table border="0" summary="Note"> 34<tr> 35<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> 36<th align="left">Note</th> 37</tr> 38<tr><td align="left" valign="top"><p> 39 The code listed here can be used by curious readers and library maintainers 40 as a reference in trying to understand the library source code. There is 41 absolutely no guarantee that the library implementation uses the exact code 42 listed here. 43 </p></td></tr> 44</table></div> 45<h4> 46<a name="boost_localfunction.implementation.h0"></a> 47 <span class="phrase"><a name="boost_localfunction.implementation.local_classes_as_template_parameters"></a></span><a class="link" href="implementation.html#boost_localfunction.implementation.local_classes_as_template_parameters">Local 48 Classes as Template Parameters</a> 49 </h4> 50<p> 51 This library uses a local class to implement the local function object. However, 52 in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a> 53 local classes (and therefore the local function objects they implement) cannot 54 be passed as template parameters (e.g., to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code> 55 algorithm), this is instead possible in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>, 56 MSVC, and some other compilers (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a> 57 and <a href="http://www.boost.org/libs/chrono" target="_top">Boost.Config</a>'s <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>). 58 To work around this limitation, this library investigated the following two 59 "tricks" (both tricks can be extended to support function default 60 parameters): 61 </p> 62<div class="orderedlist"><ol class="orderedlist" type="1"> 63<li class="listitem"> 64 The <span class="emphasis"><em>casting functor trick</em></span> uses a non-local functor 65 that calls a static member function of the local class via a function pointer. 66 The static member function then calls the correct local function body after 67 type casting the object from a <code class="computeroutput"><span class="keyword">void</span><span class="special">*</span></code> pointer (local classes can always be used 68 for type casting via <code class="computeroutput"><span class="keyword">static_cast</span></code> 69 or similar). 70 </li> 71<li class="listitem"> 72 The <span class="emphasis"><em>virtual functor trick</em></span> derives the local functor 73 class from a non-local base class. The correct overridden implementation 74 of the virtual <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 75 is then called via dynamic binding. 76 </li> 77</ol></div> 78<p> 79 For example (see also <a href="../../../example/impl_tparam_tricks.cpp" target="_top"><code class="literal">impl_tparam_tricks.cpp</code></a>): 80 </p> 81<p> 82</p> 83<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">lightweight_test</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 84<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 85<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">algorithm</span><span class="special">></span> 86 87<span class="comment">// Casting functor trick.</span> 88<span class="keyword">struct</span> <span class="identifier">casting_func</span> <span class="special">{</span> 89 <span class="keyword">explicit</span> <span class="identifier">casting_func</span><span class="special">(</span><span class="keyword">void</span><span class="special">*</span> <span class="identifier">obj</span><span class="special">,</span> <span class="keyword">void</span> <span class="special">(*</span><span class="identifier">call</span><span class="special">)(</span><span class="keyword">void</span><span class="special">*,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&))</span> 90 <span class="special">:</span> <span class="identifier">obj_</span><span class="special">(</span><span class="identifier">obj</span><span class="special">),</span> <span class="identifier">call_</span><span class="special">(</span><span class="identifier">call</span><span class="special">)</span> <span class="special">{}</span> 91 <span class="comment">// Unfortunately, function pointer call is not inlined.</span> 92 <span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">call_</span><span class="special">(</span><span class="identifier">obj_</span><span class="special">,</span> <span class="identifier">num</span><span class="special">);</span> <span class="special">}</span> 93<span class="keyword">private</span><span class="special">:</span> 94 <span class="keyword">void</span><span class="special">*</span> <span class="identifier">obj_</span><span class="special">;</span> 95 <span class="keyword">void</span> <span class="special">(*</span><span class="identifier">call_</span><span class="special">)(</span><span class="keyword">void</span><span class="special">*,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&);</span> 96<span class="special">};</span> 97 98<span class="comment">// Virtual functor trick.</span> 99<span class="keyword">struct</span> <span class="identifier">virtual_func</span> <span class="special">{</span> 100 <span class="keyword">struct</span> <span class="identifier">interface</span> <span class="special">{</span> 101 <span class="comment">// Unfortunately, virtual function call is not inlined.</span> 102 <span class="keyword">inline</span> <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&)</span> <span class="special">{}</span> 103 <span class="special">};</span> 104 <span class="keyword">explicit</span> <span class="identifier">virtual_func</span><span class="special">(</span><span class="identifier">interface</span><span class="special">&</span> <span class="identifier">func</span><span class="special">):</span> <span class="identifier">func_</span><span class="special">(&</span><span class="identifier">func</span><span class="special">)</span> <span class="special">{}</span> 105 <span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> <span class="special">(*</span><span class="identifier">func_</span><span class="special">)(</span><span class="identifier">num</span><span class="special">);</span> <span class="special">}</span> 106<span class="keyword">private</span><span class="special">:</span> 107 <span class="identifier">interface</span><span class="special">*</span> <span class="identifier">func_</span><span class="special">;</span> 108<span class="special">};</span> 109 110<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> 111 <span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> 112 113 <span class="comment">// Local class for local function.</span> 114 <span class="keyword">struct</span> <span class="identifier">local_add</span> <span class="special">:</span> <span class="identifier">virtual_func</span><span class="special">::</span><span class="identifier">interface</span> <span class="special">{</span> 115 <span class="keyword">explicit</span> <span class="identifier">local_add</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">_sum</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">_factor</span><span class="special">)</span> 116 <span class="special">:</span> <span class="identifier">sum_</span><span class="special">(</span><span class="identifier">_sum</span><span class="special">),</span> <span class="identifier">factor_</span><span class="special">(</span><span class="identifier">_factor</span><span class="special">)</span> <span class="special">{}</span> 117 <span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> 118 <span class="identifier">body</span><span class="special">(</span><span class="identifier">sum_</span><span class="special">,</span> <span class="identifier">factor_</span><span class="special">,</span> <span class="identifier">num</span><span class="special">);</span> 119 <span class="special">}</span> 120 <span class="keyword">inline</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">call</span><span class="special">(</span><span class="keyword">void</span><span class="special">*</span> <span class="identifier">obj</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> 121 <span class="identifier">local_add</span><span class="special">*</span> <span class="identifier">self</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">local_add</span><span class="special">*>(</span><span class="identifier">obj</span><span class="special">);</span> 122 <span class="identifier">self</span><span class="special">-></span><span class="identifier">body</span><span class="special">(</span><span class="identifier">self</span><span class="special">-></span><span class="identifier">sum_</span><span class="special">,</span> <span class="identifier">self</span><span class="special">-></span><span class="identifier">factor_</span><span class="special">,</span> <span class="identifier">num</span><span class="special">);</span> 123 <span class="special">}</span> 124 <span class="keyword">private</span><span class="special">:</span> 125 <span class="keyword">int</span><span class="special">&</span> <span class="identifier">sum_</span><span class="special">;</span> 126 <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">factor_</span><span class="special">;</span> 127 <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">body</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> 128 <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span> 129 <span class="special">}</span> 130 <span class="special">}</span> <span class="identifier">add_local</span><span class="special">(</span><span class="identifier">sum</span><span class="special">,</span> <span class="identifier">factor</span><span class="special">);</span> 131 <span class="identifier">casting_func</span> <span class="identifier">add_casting</span><span class="special">(&</span><span class="identifier">add_local</span><span class="special">,</span> <span class="special">&</span><span class="identifier">local_add</span><span class="special">::</span><span class="identifier">call</span><span class="special">);</span> 132 <span class="identifier">virtual_func</span> <span class="identifier">add_virtual</span><span class="special">(</span><span class="identifier">add_local</span><span class="special">);</span> 133 134 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">(</span><span class="number">10</span><span class="special">);</span> 135 <span class="identifier">std</span><span class="special">::</span><span class="identifier">fill</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="number">1</span><span class="special">);</span> 136 137 <span class="comment">// std::for_each(v.begin(), v.end(), add_local); // Error but OK on C++11.</span> 138 <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">add_casting</span><span class="special">);</span> <span class="comment">// OK.</span> 139 <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">add_virtual</span><span class="special">);</span> <span class="comment">// OK.</span> 140 141 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">200</span><span class="special">);</span> 142 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span> 143<span class="special">}</span> 144</pre> 145<p> 146 </p> 147<p> 148 The casting functor trick measured slightly better run-time performances than 149 the virtual functor trick so the current implementation of this library uses 150 the casting functor trick (probably because in addition to the indirect function 151 call, the virtual functor trick also requires accessing the <a href="http://en.wikipedia.org/wiki/Virtual_method_table" target="_top">virtual 152 function table</a>). However, neither one of the two tricks was observed 153 to allow for compiler optimizations that inline the local function calls (because 154 they rely on one indirect function call via either a function pointer or a 155 virtual function respectively). Therefore, on compilers that accept local classes 156 as template parameters (MSVC, <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>, 157 etc, see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a> 158 and <a href="http://www.boost.org/libs/chrono" target="_top">Boost.Config</a>'s <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>), 159 this library automatically generates code that passes the local class type 160 directly as template parameter without using neither one of these two tricks 161 in order to take full advantage of compiler optimizations that inline the local 162 function calls. 163 </p> 164<h4> 165<a name="boost_localfunction.implementation.h1"></a> 166 <span class="phrase"><a name="boost_localfunction.implementation.parsing_macros"></a></span><a class="link" href="implementation.html#boost_localfunction.implementation.parsing_macros">Parsing 167 Macros</a> 168 </h4> 169<p> 170 This library macros can parse the list of specified parameters and detect if 171 any of the bound variable names matches the token <code class="computeroutput"><span class="identifier">this_</span></code> 172 (to generate special code to bind the object in scope), or if the variable 173 is bound by <code class="computeroutput"><span class="keyword">const</span></code> (to generate 174 special code to bind by constant), etc. The parameter tokens are inspected 175 using preprocessor meta-programming and specifically using the macros defined 176 by the files in the <code class="literal">boost/local_function/detail/preprocessor/keyword/</code> 177 directory. <a href="#ftn.boost_localfunction.implementation.f0" class="footnote" name="boost_localfunction.implementation.f0"><sup class="footnote">[31]</sup></a> 178 </p> 179<p> 180 For example, the following code defines a macro that allows the preprocessor 181 to detect if a set of space-separated tokens ends with <code class="computeroutput"><span class="identifier">this_</span></code> 182 or not (see also <a href="../../../example/impl_pp_keyword.cpp" target="_top"><code class="literal">impl_pp_keyword.cpp</code></a>): 183 </p> 184<p> 185</p> 186<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="identifier">thisunderscore</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 187<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="keyword">const</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 188<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 189<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">lightweight_test</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 190 191<span class="comment">// Expand to 1 if space-separated tokens end with `this_`, 0 otherwise.</span> 192<span class="preprocessor">#define</span> <span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="identifier">tokens</span><span class="special">)</span> <span class="special">\</span> 193 <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_IS_THISUNDERSCORE_BACK</span><span class="special">(</span> <span class="special">\</span> 194 <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_BIND_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span> 195 <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_CONST_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span> 196 <span class="identifier">tokens</span> <span class="special">\</span> 197 <span class="special">)))</span> 198 199<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> 200 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span> 201 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span> 202 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span> 203<span class="special">}</span> 204</pre> 205<p> 206 </p> 207<div class="footnotes"> 208<br><hr style="width:100; text-align:left;margin-left: 0"> 209<div id="ftn.boost_localfunction.implementation.f0" class="footnote"><p><a href="#boost_localfunction.implementation.f0" class="para"><sup class="para">[31] </sup></a> 210 This technique is at the core of even more complex preprocessor parsing macros 211 like the ones that parse the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a> 212 syntax. 213 </p></div> 214</div> 215</div> 216<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 217<td align="left"></td> 218<td align="right"><div class="copyright-footer">Copyright © 2009-2012 Lorenzo 219 Caminiti<p> 220 Distributed under the Boost Software License, Version 1.0 (see accompanying 221 file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 222 </p> 223</div></td> 224</tr></table> 225<hr> 226<div class="spirit-nav"> 227<a accesskey="p" href="no_variadic_macros.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../reference.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 228</div> 229</body> 230</html> 231
