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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Chapter 1. Boost.LocalFunction 1.0.0</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> <link rel="next" href="boost_localfunction/getting_started.html" title="Getting Started"> </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="n" href="boost_localfunction/getting_started.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> <div class="chapter"> <div class="titlepage"><div> <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> <div><div class="legalnotice"> <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>) </p> </div></div> </div></div> <div class="toc"> <p><b>Table of Contents</b></p> <dl class="toc"> <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> <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"> <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" style="list-style-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. <a href="#ftn.boost_localfunction.introduction.f0" class="footnote" name="boost_localfunction.introduction.f0"><sup class="footnote">[1]</sup></a> </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. <a href="#ftn.boost_localfunction.introduction.f1" class="footnote" name="boost_localfunction.introduction.f1"><sup class="footnote">[2]</sup></a> </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 style="width:100; text-align:left;margin-left: 0"> <div id="ftn.boost_localfunction.introduction.f0" class="footnote"><p><a href="#boost_localfunction.introduction.f0" class="para"><sup class="para">[1] </sup></a> 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 id="ftn.boost_localfunction.introduction.f1" class="footnote"><p><a href="#boost_localfunction.introduction.f1" class="para"><sup class="para">[2] </sup></a> 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: August 11, 2020 at 15:00:10 GMT</small></p></td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"><a accesskey="n" href="boost_localfunction/getting_started.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> </body> </html>