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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_localfunction.implementation"></a><a class="link" href="implementation.html" title="Annex: Implementation">Annex: Implementation</a>
- </h2></div></div></div>
- <p>
- This section gives an overview of the key programming techniques used to implement
- this library.
- </p>
- <div class="note"><table border="0" summary="Note">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- The code listed here can be used by curious readers and library maintainers
- as a reference in trying to understand the library source code. There is
- absolutely no guarantee that the library implementation uses the exact code
- listed here.
- </p></td></tr>
- </table></div>
- <h4>
- <a name="boost_localfunction.implementation.h0"></a>
- <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
- Classes as Template Parameters</a>
- </h4>
- <p>
- This library uses a local class to implement the local function object. However,
- in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
- local classes (and therefore the local function objects they implement) cannot
- 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>
- algorithm), this is instead possible in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>,
- 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>
- 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>).
- To work around this limitation, this library investigated the following two
- "tricks" (both tricks can be extended to support function default
- parameters):
- </p>
- <div class="orderedlist"><ol class="orderedlist" type="1">
- <li class="listitem">
- The <span class="emphasis"><em>casting functor trick</em></span> uses a non-local functor
- that calls a static member function of the local class via a function pointer.
- The static member function then calls the correct local function body after
- 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
- for type casting via <code class="computeroutput"><span class="keyword">static_cast</span></code>
- or similar).
- </li>
- <li class="listitem">
- The <span class="emphasis"><em>virtual functor trick</em></span> derives the local functor
- class from a non-local base class. The correct overridden implementation
- of the virtual <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
- is then called via dynamic binding.
- </li>
- </ol></div>
- <p>
- For example (see also <a href="../../../example/impl_tparam_tricks.cpp" target="_top"><code class="literal">impl_tparam_tricks.cpp</code></a>):
- </p>
- <p>
- </p>
- <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>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">algorithm</span><span class="special">></span>
- <span class="comment">// Casting functor trick.</span>
- <span class="keyword">struct</span> <span class="identifier">casting_func</span> <span class="special">{</span>
- <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>
- <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>
- <span class="comment">// Unfortunately, function pointer call is not inlined.</span>
- <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>
- <span class="keyword">private</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>
- <span class="special">};</span>
- <span class="comment">// Virtual functor trick.</span>
- <span class="keyword">struct</span> <span class="identifier">virtual_func</span> <span class="special">{</span>
- <span class="keyword">struct</span> <span class="identifier">interface</span> <span class="special">{</span>
- <span class="comment">// Unfortunately, virtual function call is not inlined.</span>
- <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>
- <span class="special">};</span>
- <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>
- <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>
- <span class="keyword">private</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="special">};</span>
- <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>
- <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>
- <span class="comment">// Local class for local function.</span>
- <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>
- <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>
- <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>
- <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">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>
- <span class="special">}</span>
- <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>
- <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>
- <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>
- <span class="special">}</span>
- <span class="keyword">private</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">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>
- <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>
- <span class="special">}</span>
- <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>
- <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>
- <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>
- <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>
- <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>
- <span class="comment">// std::for_each(v.begin(), v.end(), add_local); // Error but OK on C++11.</span>
- <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>
- <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>
- <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>
- <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- <p>
- The casting functor trick measured slightly better run-time performances than
- the virtual functor trick so the current implementation of this library uses
- the casting functor trick (probably because in addition to the indirect function
- call, the virtual functor trick also requires accessing the <a href="http://en.wikipedia.org/wiki/Virtual_method_table" target="_top">virtual
- function table</a>). However, neither one of the two tricks was observed
- to allow for compiler optimizations that inline the local function calls (because
- they rely on one indirect function call via either a function pointer or a
- virtual function respectively). Therefore, on compilers that accept local classes
- as template parameters (MSVC, <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>,
- etc, see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a>
- 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>),
- this library automatically generates code that passes the local class type
- directly as template parameter without using neither one of these two tricks
- in order to take full advantage of compiler optimizations that inline the local
- function calls.
- </p>
- <h4>
- <a name="boost_localfunction.implementation.h1"></a>
- <span class="phrase"><a name="boost_localfunction.implementation.parsing_macros"></a></span><a class="link" href="implementation.html#boost_localfunction.implementation.parsing_macros">Parsing
- Macros</a>
- </h4>
- <p>
- This library macros can parse the list of specified parameters and detect if
- any of the bound variable names matches the token <code class="computeroutput"><span class="identifier">this_</span></code>
- (to generate special code to bind the object in scope), or if the variable
- is bound by <code class="computeroutput"><span class="keyword">const</span></code> (to generate
- special code to bind by constant), etc. The parameter tokens are inspected
- using preprocessor meta-programming and specifically using the macros defined
- by the files in the <code class="literal">boost/local_function/detail/preprocessor/keyword/</code>
- directory. <a href="#ftn.boost_localfunction.implementation.f0" class="footnote" name="boost_localfunction.implementation.f0"><sup class="footnote">[31]</sup></a>
- </p>
- <p>
- For example, the following code defines a macro that allows the preprocessor
- to detect if a set of space-separated tokens ends with <code class="computeroutput"><span class="identifier">this_</span></code>
- or not (see also <a href="../../../example/impl_pp_keyword.cpp" target="_top"><code class="literal">impl_pp_keyword.cpp</code></a>):
- </p>
- <p>
- </p>
- <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>
- <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>
- <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>
- <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>
- <span class="comment">// Expand to 1 if space-separated tokens end with `this_`, 0 otherwise.</span>
- <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>
- <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_IS_THISUNDERSCORE_BACK</span><span class="special">(</span> <span class="special">\</span>
- <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_BIND_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span>
- <span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_CONST_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span>
- <span class="identifier">tokens</span> <span class="special">\</span>
- <span class="special">)))</span>
- <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>
- <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>
- <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>
- <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- <div class="footnotes">
- <br><hr style="width:100; text-align:left;margin-left: 0">
- <div id="ftn.boost_localfunction.implementation.f0" class="footnote"><p><a href="#boost_localfunction.implementation.f0" class="para"><sup class="para">[31] </sup></a>
- This technique is at the core of even more complex preprocessor parsing macros
- like the ones that parse the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a>
- syntax.
- </p></div>
- </div>
- </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 © 2009-2012 Lorenzo
- Caminiti<p>
- Distributed under the Boost Software License, Version 1.0 (see accompanying
- 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>)
- </p>
- </div></td>
- </tr></table>
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