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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">&lt;</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">&gt;</span>
84<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
85<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">algorithm</span><span class="special">&gt;</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">&amp;))</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">&amp;</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">&amp;);</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">&amp;)</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">&amp;</span> <span class="identifier">func</span><span class="special">):</span> <span class="identifier">func_</span><span class="special">(&amp;</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">&amp;</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">&amp;</span> <span class="identifier">_sum</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</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">&amp;</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">&amp;</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">&lt;</span><span class="identifier">local_add</span><span class="special">*&gt;(</span><span class="identifier">obj</span><span class="special">);</span>
122 <span class="identifier">self</span><span class="special">-&gt;</span><span class="identifier">body</span><span class="special">(</span><span class="identifier">self</span><span class="special">-&gt;</span><span class="identifier">sum_</span><span class="special">,</span> <span class="identifier">self</span><span class="special">-&gt;</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">&amp;</span> <span class="identifier">sum_</span><span class="special">;</span>
126 <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</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">&amp;</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">factor</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</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">(&amp;</span><span class="identifier">add_local</span><span class="special">,</span> <span class="special">&amp;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&lt;</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">&gt;</span>
187<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
188<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
189<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">&amp;</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 &#169; 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>
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