Boost C++ Libraries

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Chapter 27. Boost.PolyCollection

Joaquín M López Muñoz

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

Table of Contents

Introduction
An efficient polymorphic data structure
Tutorial
Basics
Deeper into the segmented nature of Boost.PolyCollection
Insertion and emplacement
Exceptions
Algorithms
Performance
Container definitions
Insertion tests
Processing tests
Reference
Polymorphism models
Polymorphic containers
Header "boost/poly_collection/exception.hpp" synopsis
Header "boost/poly_collection/base_collection_fwd.hpp" synopsis
Header "boost/poly_collection/base_collection.hpp" synopsis
Header "boost/poly_collection/function_collection_fwd.hpp" synopsis
Header "boost/poly_collection/function_collection.hpp" synopsis
Header "boost/poly_collection/any_collection_fwd.hpp" synopsis
Header "boost/poly_collection/any_collection.hpp" synopsis
Header "boost/poly_collection/algorithm.hpp" synopsis
Future work
Alternative RTTI systems
Copy traits
Parallel algorithms
variant_collection
Ordered polymorphic collections
Release notes
Boost 1.65
Acknowledgments

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 type and size 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 in two types of inefficiency:

  • The lack of memory contiguity produced by heap allocation degrades CPU cache performance.
  • Executing virtual operations on a sequence of polymorphic objects whose actual types differ from one to the next results in failures in branch prediction and a lower execution speed.

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:

  • boost::base_collection
  • boost::function_collection
  • boost::any_collection

respectively dealing with three different types of dynamic polymorphism available in C++:

The interface of these containers closely follows that of standard containers. Additionally, the library provides versions of many of the standard library algorithms (including std::for_each) with improved performance and a special feature called type restitution 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 devirtualization.

[Note] Note

Boost.PolyCollection is a header-only library. C++11 support is required. The library has been verified to work with Visual Studio 2015, GCC 5.2 and Clang 3.7.

Last revised: August 30, 2017 at 21:54:55 GMT


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