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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>Chapter 28. Boost.PolyCollection</title>
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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="poly_collection"></a>Chapter 28. Boost.PolyCollection</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Joaquín M</span> <span class="surname">López Muñoz</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2016-2019 Joaquín
M López Muñoz</p></div>
<div><div class="legalnotice">
<a name="poly_collection.legal"></a><p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="poly_collection.html#poly_collection.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="poly_collection/an_efficient_polymorphic_data_st.html">An efficient
polymorphic data structure</a></span></dt>
<dt><span class="section"><a href="poly_collection/tutorial.html">Tutorial</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="poly_collection/tutorial.html#poly_collection.tutorial.basics">Basics</a></span></dt>
<dt><span class="section"><a href="poly_collection/tutorial.html#poly_collection.tutorial.deeper_into_the_segmented_nature">Deeper
into the segmented nature of Boost.PolyCollection</a></span></dt>
<dt><span class="section"><a href="poly_collection/tutorial.html#poly_collection.tutorial.insertion_and_emplacement">Insertion
and emplacement</a></span></dt>
<dt><span class="section"><a href="poly_collection/tutorial.html#poly_collection.tutorial.exceptions">Exceptions</a></span></dt>
<dt><span class="section"><a href="poly_collection/tutorial.html#poly_collection.tutorial.algorithms">Algorithms</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="poly_collection/performance.html">Performance</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="poly_collection/performance.html#poly_collection.performance.container_definitions">Container
definitions</a></span></dt>
<dt><span class="section"><a href="poly_collection/performance.html#poly_collection.performance.insertion_tests">Insertion
tests</a></span></dt>
<dt><span class="section"><a href="poly_collection/performance.html#poly_collection.performance.processing_tests">Processing
tests</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="poly_collection/reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.polymorphism_models">Polymorphism
models</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.polymorphic_containers">Polymorphic
containers</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_exc">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/exception.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_bas">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/base_collection_fwd.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_ba0">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/base_collection.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_fun">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/function_collection_fwd.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_fu0">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/function_collection.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_any">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/any_collection_fwd.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_an0">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/any_collection.hpp"</span></code>
synopsis</a></span></dt>
<dt><span class="section"><a href="poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_alg">Header
<code class="computeroutput"><span class="string">"boost/poly_collection/algorithm.hpp"</span></code>
synopsis</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="poly_collection/future_work.html">Future work</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="poly_collection/future_work.html#poly_collection.future_work.alternative_rtti_systems">Alternative
RTTI systems</a></span></dt>
<dt><span class="section"><a href="poly_collection/future_work.html#poly_collection.future_work.copy_traits">Copy traits</a></span></dt>
<dt><span class="section"><a href="poly_collection/future_work.html#poly_collection.future_work.parallel_algorithms">Parallel
algorithms</a></span></dt>
<dt><span class="section"><a href="poly_collection/future_work.html#poly_collection.future_work.variant_collection"><code class="computeroutput"><span class="identifier">variant_collection</span></code></a></span></dt>
<dt><span class="section"><a href="poly_collection/future_work.html#poly_collection.future_work.ordered_polymorphic_collections">Ordered
polymorphic collections</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="poly_collection/release_notes.html">Release notes</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_71">Boost 1.71</a></span></dt>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_70">Boost 1.70</a></span></dt>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_69">Boost 1.69</a></span></dt>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_67">Boost 1.67</a></span></dt>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_66">Boost 1.66</a></span></dt>
<dt><span class="section"><a href="poly_collection/release_notes.html#poly_collection.release_notes.boost_1_65">Boost 1.65</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="poly_collection/acknowledgments.html">Acknowledgments</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="poly_collection.introduction"></a><a class="link" href="poly_collection.html#poly_collection.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<p>
Dynamic polymorphism in C++ requires that objects (such as instances of classes
derived from an abstract base) be accessed through an indirection pointer because
their actual <span class="emphasis"><em>type</em></span> and <span class="emphasis"><em>size</em></span> are not
known at the point of usage. As a consequence, regular containers cannot store
polymorphic objects directly: the usual workaround is to have containers of
pointers to heap-allocated elements. In modern computer architectures this
pattern incurs two types of inefficiency:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The lack of memory contiguity produced by heap allocation degrades CPU
cache performance.
</li>
<li class="listitem">
Executing virtual operations on a sequence of polymorphic objects whose
actual types differ from one to the next results in failures in <a href="https://en.wikipedia.org/wiki/Branch_predictor" target="_top">branch prediction</a>
and a lower execution speed.
</li>
</ul></div>
<p>
When the particular traversal order is not relevant to the user application,
Boost.PolyCollection proposes an alternative data structure that restores memory
contiguity and packs elements according to their concrete type. Three container
class templates are provided:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_collection</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function_collection</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">any_collection</span></code>
</li>
</ul></div>
<p>
respectively dealing with three different types of dynamic polymorphism available
in C++:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Classic base/derived or OOP polymorphism.
</li>
<li class="listitem">
Function wrapping in the spirit of <a href="http://en.cppreference.com/w/cpp/utility/functional/function" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">function</span></code></a>.
</li>
<li class="listitem">
So-called <a href="https://en.wikipedia.org/wiki/Duck_typing" target="_top"><span class="emphasis"><em>duck
typing</em></span></a> as implemented by <a href="../../libs/type_erasure" target="_top">Boost.TypeErasure</a>.
</li>
</ul></div>
<p>
The interface of these containers closely follows that of standard containers.
Additionally, the library provides versions of many of the standard library
algorithms (including <a href="http://en.cppreference.com/w/cpp/algorithm/for_each" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code></a>)
with improved performance and a special feature called <span class="emphasis"><em>type restitution</em></span>
that allows user code to provide clues on the concrete types of the elements
stored for further opportunities of increased efficiency related to inlining
and <a href="http://hubicka.blogspot.com.es/2014/01/devirtualization-in-c-part-1.html" target="_top"><span class="emphasis"><em>devirtualization</em></span></a>.
</p>
<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"><p>
Boost.PolyCollection is a header-only library. C++11 support is required.
The library has been verified to work with Visual Studio 2015, GCC 4.8 and
Clang 3.3.
</p></td></tr>
</table></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 14, 2019 at 12:09:36 GMT</small></p></td>
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