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<title>Chapter 1. Boost.LocalFunction 1.0.0</title>
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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>
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<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>
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<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>
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