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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Extended functionality: Basic extensions</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="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../container.html" title="Chapter 9. Boost.Container"> <link rel="prev" href="non_standard_containers.html" title="Non-standard containers"> <link rel="next" href="configurable_containers.html" title="Extended functionality: Configurable containers"> </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="p" href="non_standard_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../container.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="configurable_containers.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="container.extended_functionality"></a><a class="link" href="extended_functionality.html" title="Extended functionality: Basic extensions">Extended functionality: Basic extensions</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.default_initialialization">Default initialization for vector-like containers</a></span></dt> <dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.ordered_range_insertion">Ordered range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>, <span class="emphasis"><em>ordered_range</em></span>) </a></span></dt> <dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.constant_time_range_splice">Constant-time range splice for <code class="computeroutput"><span class="special">(</span><span class="identifier">s</span><span class="special">)</span><span class="identifier">list</span></code></a></span></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="container.extended_functionality.default_initialialization"></a><a class="link" href="extended_functionality.html#container.extended_functionality.default_initialialization" title="Default initialization for vector-like containers">Default initialization for vector-like containers</a> </h3></div></div></div> <p> STL and most other containers value initialize new elements in common operations like <code class="computeroutput"><span class="identifier">vector</span><span class="special">::</span><span class="identifier">resize</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code> or <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">vector</span><span class="special">::</span><span class="identifier">vector</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code>. </p> <p> In some performance-sensitive environments, where vectors are used as a replacement for variable-size buffers for file or network operations, <a href="http://en.cppreference.com/w/cpp/language/value_initialization" target="_top">value initialization</a> is a cost that is not negligible as elements are going to be overwritten by an external source shortly after new elements are added to the container. </p> <p> <span class="bold"><strong>Boost.Container</strong></span> offers two new members for <code class="computeroutput"><span class="identifier">vector</span></code>, <code class="computeroutput"><span class="identifier">static_vector</span></code> and <code class="computeroutput"><span class="identifier">stable_vector</span></code>: <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">container</span><span class="special">::</span><span class="identifier">container</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">,</span> <span class="identifier">default_init_t</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">container</span><span class="special">::</span><span class="identifier">resize</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">,</span> <span class="identifier">default_init_t</span><span class="special">)</span></code>, where new elements are constructed using <a href="http://en.cppreference.com/w/cpp/language/default_initialization" target="_top">default initialization</a>. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="container.extended_functionality.ordered_range_insertion"></a><a class="link" href="extended_functionality.html#container.extended_functionality.ordered_range_insertion" title="Ordered range insertion for associative containers (ordered_unique_range, ordered_range)">Ordered range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>, <span class="emphasis"><em>ordered_range</em></span>) </a> </h3></div></div></div> <p> When filling associative containers big performance gains can be achieved if the input range to be inserted is guaranteed by the user to be ordered according to the predicate. This can happen when inserting values from a <code class="computeroutput"><span class="identifier">set</span></code> to a <code class="computeroutput"><span class="identifier">multiset</span></code> or between different associative container families (<code class="computeroutput"><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span><span class="special">/</span><span class="identifier">map</span></code> vs. <code class="computeroutput"><span class="identifier">flat_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span><span class="special">/</span><span class="identifier">map</span></code>). </p> <p> <span class="bold"><strong>Boost.Container</strong></span> has some overloads for constructors and insertions taking an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code> or an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> tag parameters as the first argument. When an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code> overload is used, the user notifies the container that the input range is ordered according to the container predicate and has no duplicates. When an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> overload is used, the user notifies the container that the input range is ordered according to the container predicate but it might have duplicates. With this information, the container can avoid multiple predicate calls and improve insertion times. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="container.extended_functionality.constant_time_range_splice"></a><a class="link" href="extended_functionality.html#container.extended_functionality.constant_time_range_splice" title="Constant-time range splice for (s)list">Constant-time range splice for <code class="computeroutput"><span class="special">(</span><span class="identifier">s</span><span class="special">)</span><span class="identifier">list</span></code></a> </h3></div></div></div> <p> In the first C++ standard <code class="computeroutput"><span class="identifier">list</span><span class="special">::</span><span class="identifier">size</span><span class="special">()</span></code> was not required to be constant-time, and that caused some controversy in the C++ community. Quoting Howard Hinnant's <a href="http://howardhinnant.github.io/On_list_size.html" target="_top"><span class="emphasis"><em>On List Size</em></span></a> paper: </p> <div class="blockquote"><blockquote class="blockquote"> <p> <span class="emphasis"><em>There is a considerable debate on whether <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">size</span><span class="special">()</span></code> should be O(1) or O(N). The usual argument notes that it is a tradeoff with:</em></span> </p> <p> <code class="computeroutput"><span class="identifier">splice</span><span class="special">(</span><span class="identifier">iterator</span> <span class="identifier">position</span><span class="special">,</span> <span class="identifier">list</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">iterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">);</span></code> </p> <p> <span class="emphasis"><em>If size() is O(1) and this != &x, then this method must perform a linear operation so that it can adjust the size member in each list</em></span> </p> </blockquote></div> <p> C++11 definitely required <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code> to be O(1), so range splice became O(N). However, Howard Hinnant's paper proposed a new <code class="computeroutput"><span class="identifier">splice</span></code> overload so that even O(1) <code class="computeroutput"><span class="identifier">list</span><span class="special">:</span><span class="identifier">size</span><span class="special">()</span></code> implementations could achieve O(1) range splice when the range size was known to the caller: </p> <div class="blockquote"><blockquote class="blockquote"> <p> <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">splice</span><span class="special">(</span><span class="identifier">iterator</span> <span class="identifier">position</span><span class="special">,</span> <span class="identifier">list</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">iterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">);</span></code> </p> <p> <span class="bold"><strong>Effects</strong></span>: Inserts elements in the range [first, last) before position and removes the elements from x. </p> <p> <span class="bold"><strong>Requires</strong></span>: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Invalidates only the iterators and references to the spliced elements. n == distance(first, last). </p> <p> <span class="bold"><strong>Throws</strong></span>: Nothing. </p> <p> <span class="bold"><strong>Complexity</strong></span>: Constant time. </p> </blockquote></div> <p> This new splice signature allows the client to pass the distance of the input range in. This information is often available at the call site. If it is passed in, then the operation is constant time, even with an O(1) size. </p> <p> <span class="bold"><strong>Boost.Container</strong></span> implements this overload for <code class="computeroutput"><span class="identifier">list</span></code> and a modified version of it for <code class="computeroutput"><span class="identifier">slist</span></code> (as <code class="computeroutput"><span class="identifier">slist</span><span class="special">::</span><span class="identifier">size</span><span class="special">()</span></code> is also <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>). </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2009-2018 Ion Gaztanaga<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> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="non_standard_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../container.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="configurable_containers.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>