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| <div><h2 class="title"> |
| <a name="boost_localfunction"></a>Chapter 1. Boost.LocalFunction 1.0.0</h2></div> |
| <div><div class="author"><h3 class="author"> |
| <span class="firstname">Lorenzo</span> <span class="surname">Caminiti <code class="email"><<a class="email" href="mailto:lorcaminiti@gmail.com">lorcaminiti@gmail.com</a>></code></span> |
| </h3></div></div> |
| <div><p class="copyright">Copyright © 2009-2012 Lorenzo |
| Caminiti</p></div> |
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| <a name="boost_localfunction.legal"></a><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>) |
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| <div class="toc"> |
| <p><b>Table of Contents</b></p> |
| <dl> |
| <dt><span class="section"><a href="index.html#boost_localfunction.introduction">Introduction</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/getting_started.html">Getting Started</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.this_documentation">This |
| Documentation</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.compilers_and_platforms">Compilers |
| and Platforms</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.installation">Installation</a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="boost_localfunction/tutorial.html">Tutorial</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.local_functions">Local Functions</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.Binding">Binding Variables</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.binding_the_object__this_">Binding |
| the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.templates">Templates</a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html">Advanced Topics</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.default_parameters">Default |
| Parameters</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros">Commas |
| and Symbols in Macros</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns">Assignments |
| and Returns</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.nesting">Nesting</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_">Accessing |
| Types (concepts, etc)</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_">Specifying |
| Types (no Boost.Typeof)</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.inlining">Inlining</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.recursion">Recursion</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.overloading">Overloading</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications">Exception |
| Specifications</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers">Storage |
| Classifiers</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions">Same |
| Line Expansions</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_">Limitations |
| (operators, etc)</a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="boost_localfunction/examples.html">Examples</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_">GCC |
| Lambdas (without C++11)</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.constant_blocks">Constant |
| Blocks</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.scope_exits">Scope Exits</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.boost_phoenix_functions">Boost.Phoenix |
| Functions</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.closures">Closures</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_nested_functions">GCC |
| Nested Functions</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.n_papers">N-Papers</a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="boost_localfunction/alternatives.html">Annex: Alternatives</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/no_variadic_macros.html">Annex: No Variadic |
| Macros</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/implementation.html">Annex: Implementation</a></span></dt> |
| <dt><span class="section"><a href="reference.html">Reference</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="reference.html#header.boost.local_function_hpp">Header <boost/local_function.hpp></a></span></dt> |
| <dt><span class="section"><a href="reference.html#header.boost.local_function.config_hpp">Header <boost/local_function/config.hpp></a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="boost_localfunction/release_notes.html">Release Notes</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/bibliography.html">Bibliography</a></span></dt> |
| <dt><span class="section"><a href="boost_localfunction/acknowledgments.html">Acknowledgments</a></span></dt> |
| </dl> |
| </div> |
| <p> |
| This library allows to program functions locally, within other functions, and |
| directly within the scope where they are needed. |
| </p> |
| <div class="section boost_localfunction_introduction"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_localfunction.introduction"></a><a class="link" href="index.html#boost_localfunction.introduction" title="Introduction">Introduction</a> |
| </h2></div></div></div> |
| <p> |
| <span class="emphasis"><em>Local functions</em></span> (a.k.a., <a href="http://en.wikipedia.org/wiki/Nested_function" target="_top"><span class="emphasis"><em>nested |
| functions</em></span></a>) are a form of <span class="emphasis"><em>information hiding</em></span> |
| and they are useful for dividing procedural tasks into subtasks which are only |
| meaningful locally, avoiding cluttering other parts of the program with functions, |
| variables, etc unrelated to those parts. Therefore, local functions complement |
| other structuring possibilities such as namespaces and classes. Local functions |
| are a feature of many programming languages, notably <a href="http://en.wikipedia.org/wiki/Nested_function#An_example" target="_top">Pascal</a> |
| and <a href="http://en.wikipedia.org/wiki/Nesting_(computing)#In_programming" target="_top">Ada</a>, |
| yet lacking from <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a> |
| (see also <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2511.html" target="_top">[N2511]</a>). |
| </p> |
| <p> |
| Using <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 |
| lambda functions</a>, it is possible to implement local functions by naming |
| lambda functions assigning them to local variables. For example (see also |
| <a href="../../example/add_cxx11_lambda.cpp" target="_top"><code class="literal">add_cxx11_lambda.cpp</code></a>): |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><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="comment">// Some local scope.</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">// Variables in scope to bind.</span> |
| |
| <span class="keyword">auto</span> <span class="identifier">add</span> <span class="special">=</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="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// C++11 only.</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="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Call the lambda.</span> |
| <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span> <span class="comment">// Pass it to an algorithm.</span> |
| |
| <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span> <span class="comment">// Assert final summation value.</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> |
| This library allows to program local functions portably between <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a> |
| and <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> (and |
| with performances comparable to lambda functions on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> |
| compilers). For example (see also <a href="../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>): |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><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="comment">// Some local scope.</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">// Variables in scope to bind.</span> |
| |
| <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</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="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span> |
| |
| <span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Call the local function.</span> |
| <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span> <span class="comment">// Pass it to an algorithm.</span> |
| |
| <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span> <span class="comment">// Assert final summation value.</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> |
| This library supports the following features for local functions: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Local functions can capture, or better <a href="http://en.wikipedia.org/wiki/Name_binding" target="_top"><span class="emphasis"><em>bind</em></span></a>, |
| any of the variables from the enclosing scope (a function together with |
| its captured variables is also called a <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)" target="_top"><span class="emphasis"><em>closure</em></span></a>). |
| </li> |
| <li class="listitem"> |
| The local function body is programmed using the usual C++ statement syntax |
| (as a consequence, compiler errors and debugging retain their usual meaning |
| and format). |
| </li> |
| <li class="listitem"> |
| Local functions can be passed as template parameters so they can be conveniently |
| used with STL algorithms and other templates. <sup>[<a name="boost_localfunction.introduction.f0" href="#ftn.boost_localfunction.introduction.f0" class="footnote">1</a>]</sup> |
| </li> |
| <li class="listitem"> |
| However, local functions must be specified within a declarative context |
| (e.g., at a point in the code where local variables can be declared) thus |
| they cannot be specified within expressions. <sup>[<a name="boost_localfunction.introduction.f1" href="#ftn.boost_localfunction.introduction.f1" class="footnote">2</a>]</sup> |
| </li> |
| </ul></div> |
| <p> |
| See the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a> |
| section for a comparison between this library, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 |
| lambda functions</a>, <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>, |
| and other C++ techniques that implement features related to local functions. |
| </p> |
| </div> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.introduction.f0" href="#boost_localfunction.introduction.f0" class="para">1</a>] </sup> |
| This is a strength with respect to <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a> |
| functors implemented using local classes which cannot be passed as template |
| parameters (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a> |
| and the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a> |
| section). |
| </p></div> |
| <div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.introduction.f1" href="#boost_localfunction.introduction.f1" class="para">2</a>] </sup> |
| This is a weakness with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 |
| lambda functions</a> which can instead be specified also within expressions |
| (see the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a> |
| section). |
| </p></div> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"><p><small>Last revised: April 28, 2012 at 02:07:02 GMT</small></p></td> |
| <td align="right"><div class="copyright-footer"></div></td> |
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