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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Annex: Implementation</title> <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> <link rel="home" href="../index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> <link rel="up" href="../index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> <link rel="prev" href="no_variadic_macros.html" title="Annex: No Variadic Macros"> <link rel="next" href="../reference.html" title="Reference"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> <td align="center"><a href="../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> <div class="section boost_localfunction_implementation"> <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> This section gives an overview of the key programming techniques used to implement this library. <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"> 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. </td></tr> </table></div> <h4> <a name="boost_localfunction.implementation.h0"></a> <a name="boost_localfunction.implementation.local_classes_as_template_parameters"></a><a class="link" href="implementation.html#boost_localfunction.implementation.local_classes_as_template_parameters">Local Classes as Template Parameters</a> </h4> 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">std::for_each</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">BOOST_NO_LOCAL_CLASS_TEMPLATE_PARAMETERS</code>). To work around this limitation, this library investigated the following two "tricks" (both tricks can be extended to support function default parameters): <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem"> The casting functor trick 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">void*</code> pointer (local classes can always be used for type casting via <code class="computeroutput">static_cast</code> or similar). </li> <li class="listitem"> The virtual functor trick derives the local functor class from a non-local base class. The correct overridden implementation of the virtual <code class="computeroutput">operator()</code> is then called via dynamic binding. </li> </ol></div> For example (see also <a href="../../../example/impl_tparam_tricks.cpp" target="_top"><code class="literal">impl_tparam_tricks.cpp</code></a>): <pre class="programlisting">#include <boost/detail/lightweight_test.hpp> #include <vector> #include <algorithm> // Casting functor trick. struct casting_func { explicit casting_func(void* obj, void (*call)(void*, const int&)) : obj_(obj), call_(call) {} // Unfortunately, function pointer call is not inlined. inline void operator()(const int& num) { call_(obj_, num); } private: void* obj_; void (*call_)(void*, const int&); }; // Virtual functor trick. struct virtual_func { struct interface { // Unfortunately, virtual function call is not inlined. inline virtual void operator()(const int&) {} }; explicit virtual_func(interface& func): func_(&func) {} inline void operator()(const int& num) { (*func_)(num); } private: interface* func_; }; int main(void) { int sum = 0, factor = 10; // Local class for local function. struct local_add : virtual_func::interface { explicit local_add(int& _sum, const int& _factor) : sum_(_sum), factor_(_factor) {} inline void operator()(const int& num) { body(sum_, factor_, num); } inline static void call(void* obj, const int& num) { local_add* self = static_cast<local_add*>(obj); self->body(self->sum_, self->factor_, num); } private: int& sum_; const int& factor_; inline void body(int& sum, const int& factor, const int& num) { sum += factor * num; } } add_local(sum, factor); casting_func add_casting(&add_local, &local_add::call); virtual_func add_virtual(add_local); std::vector<int> v(10); std::fill(v.begin(), v.end(), 1); // std::for_each(v.begin(), v.end(), add_local); // Error but OK on C++11. std::for_each(v.begin(), v.end(), add_casting); // OK. std::for_each(v.begin(), v.end(), add_virtual); // OK. BOOST_TEST(sum == 200); return boost::report_errors(); } </pre> 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">BOOST_NO_LOCAL_CLASS_TEMPLATE_PARAMETERS</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. <h4> <a name="boost_localfunction.implementation.h1"></a> <a name="boost_localfunction.implementation.parsing_macros"></a><a class="link" href="implementation.html#boost_localfunction.implementation.parsing_macros">Parsing Macros</a> </h4> 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">this_</code> (to generate special code to bind the object in scope), or if the variable is bound by <code class="computeroutput">const</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 name="boost_localfunction.implementation.f0" href="#ftn.boost_localfunction.implementation.f0" class="footnote">31</a>] 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">this_</code> or not (see also <a href="../../../example/impl_pp_keyword.cpp" target="_top"><code class="literal">impl_pp_keyword.cpp</code></a>): <pre class="programlisting">#include <boost/local_function/detail/preprocessor/keyword/thisunderscore.hpp> #include <boost/local_function/detail/preprocessor/keyword/const.hpp> #include <boost/local_function/detail/preprocessor/keyword/bind.hpp> #include <boost/detail/lightweight_test.hpp> // Expand to 1 if space-separated tokens end with `this_`, 0 otherwise. #define IS_THIS_BACK(tokens) \ BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_IS_THISUNDERSCORE_BACK( \ BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_BIND_REMOVE_FRONT( \ BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_CONST_REMOVE_FRONT( \ tokens \ ))) int main(void) { BOOST_TEST(IS_THIS_BACK(const bind this_) == 1); BOOST_TEST(IS_THIS_BACK(const bind& x) == 0); return boost::report_errors(); } </pre> <div class="footnotes"> <hr width="100" align="left"> <div class="footnote">[<a id="ftn.boost_localfunction.implementation.f0" href="#boost_localfunction.implementation.f0" class="para">31</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. </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 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>) </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>