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<div><h2 class="title">
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<a name="boost_localfunction"></a>Chapter 1. Boost.LocalFunction 1.0.0</h2></div>
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<div><div class="author"><h3 class="author">
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<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>
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<div><p class="copyright">Copyright © 2009-2012 Lorenzo
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Caminiti</p></div>
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<a name="boost_localfunction.legal"></a><p>
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Distributed under the Boost Software License, Version 1.0 (see accompanying
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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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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="index.html#boost_localfunction.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/getting_started.html">Getting Started</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.this_documentation">This
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Documentation</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.compilers_and_platforms">Compilers
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and Platforms</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.installation">Installation</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_localfunction/tutorial.html">Tutorial</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.local_functions">Local Functions</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.Binding">Binding Variables</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.binding_the_object__this_">Binding
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the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.templates">Templates</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html">Advanced Topics</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.default_parameters">Default
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Parameters</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros">Commas
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and Symbols in Macros</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns">Assignments
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and Returns</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.nesting">Nesting</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_">Accessing
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Types (concepts, etc)</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_">Specifying
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Types (no Boost.Typeof)</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.inlining">Inlining</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.recursion">Recursion</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.overloading">Overloading</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications">Exception
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Specifications</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers">Storage
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Classifiers</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions">Same
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Line Expansions</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_">Limitations
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(operators, etc)</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_localfunction/examples.html">Examples</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_">GCC
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Lambdas (without C++11)</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.constant_blocks">Constant
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Blocks</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.scope_exits">Scope Exits</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.boost_phoenix_functions">Boost.Phoenix
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Functions</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.closures">Closures</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_nested_functions">GCC
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Nested Functions</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.n_papers">N-Papers</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_localfunction/alternatives.html">Annex: Alternatives</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/no_variadic_macros.html">Annex: No Variadic
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Macros</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/implementation.html">Annex: Implementation</a></span></dt>
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<dt><span class="section"><a href="reference.html">Reference</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="reference.html#header.boost.local_function_hpp">Header <boost/local_function.hpp></a></span></dt>
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<dt><span class="section"><a href="reference.html#header.boost.local_function.config_hpp">Header <boost/local_function/config.hpp></a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_localfunction/release_notes.html">Release Notes</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/bibliography.html">Bibliography</a></span></dt>
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<dt><span class="section"><a href="boost_localfunction/acknowledgments.html">Acknowledgments</a></span></dt>
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</dl>
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</div>
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<p>
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This library allows to program functions locally, within other functions, and
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directly within the scope where they are needed.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_localfunction.introduction"></a><a class="link" href="index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
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</h2></div></div></div>
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<p>
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<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
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functions</em></span></a>) are a form of <span class="emphasis"><em>information hiding</em></span>
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and they are useful for dividing procedural tasks into subtasks which are only
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meaningful locally, avoiding cluttering other parts of the program with functions,
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variables, etc unrelated to those parts. Therefore, local functions complement
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other structuring possibilities such as namespaces and classes. Local functions
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are a feature of many programming languages, notably <a href="http://en.wikipedia.org/wiki/Nested_function#An_example" target="_top">Pascal</a>
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and <a href="http://en.wikipedia.org/wiki/Nesting_(computing)#In_programming" target="_top">Ada</a>,
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yet lacking from <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
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(see also <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2511.html" target="_top">[N2511]</a>).
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</p>
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<p>
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Using <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
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lambda functions</a>, it is possible to implement local functions by naming
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lambda functions assigning them to local variables. For example (see also
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<a href="../../example/add_cxx11_lambda.cpp" target="_top"><code class="literal">add_cxx11_lambda.cpp</code></a>):
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</p>
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<p>
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</p>
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<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>
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<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>
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<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>
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<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>
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<span class="special">};</span>
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<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>
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<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>
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<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>
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<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>
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<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
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<span class="special">}</span>
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</pre>
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<p>
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</p>
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<p>
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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>
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and <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> (and
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with performances comparable to lambda functions on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
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compilers). For example (see also <a href="../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>):
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</p>
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<p>
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</p>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
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<span class="special">}</span>
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</pre>
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<p>
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</p>
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<p>
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This library supports the following features for local functions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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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>,
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any of the variables from the enclosing scope (a function together with
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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>).
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</li>
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<li class="listitem">
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The local function body is programmed using the usual C++ statement syntax
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(as a consequence, compiler errors and debugging retain their usual meaning
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and format).
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</li>
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<li class="listitem">
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Local functions can be passed as template parameters so they can be conveniently
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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>
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</li>
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<li class="listitem">
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However, local functions must be specified within a declarative context
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(e.g., at a point in the code where local variables can be declared) thus
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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>
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</li>
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</ul></div>
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<p>
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See the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
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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
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lambda functions</a>, <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>,
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and other C++ techniques that implement features related to local functions.
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</p>
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</div>
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<div class="footnotes">
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<br><hr style="width:100; text-align:left;margin-left: 0">
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<div id="ftn.boost_localfunction.introduction.f0" class="footnote"><p><a href="#boost_localfunction.introduction.f0" class="para"><sup class="para">[1] </sup></a>
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This is a strength with respect to <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
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functors implemented using local classes which cannot be passed as template
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parameters (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a>
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and the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
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section).
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</p></div>
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<div id="ftn.boost_localfunction.introduction.f1" class="footnote"><p><a href="#boost_localfunction.introduction.f1" class="para"><sup class="para">[2] </sup></a>
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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
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lambda functions</a> which can instead be specified also within expressions
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(see the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
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section).
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</p></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"><p><small>Last revised: October 14, 2015 at 23:57:30 GMT</small></p></td>
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<td align="right"><div class="copyright-footer"></div></td>
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