1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Extended functionality: Basic extensions</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../container.html" title="Chapter 9. Boost.Container"> 10<link rel="prev" href="non_standard_containers.html" title="Non-standard containers"> 11<link rel="next" href="configurable_containers.html" title="Extended functionality: Configurable containers"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="container.extended_functionality"></a><a class="link" href="extended_functionality.html" title="Extended functionality: Basic extensions">Extended functionality: 29 Basic extensions</a> 30</h2></div></div></div> 31<div class="toc"><dl class="toc"> 32<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.default_initialialization">Default 33 initialization for vector-like containers</a></span></dt> 34<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.ordered_range_insertion">Ordered 35 range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>, 36 <span class="emphasis"><em>ordered_range</em></span>) </a></span></dt> 37<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.constant_time_range_splice">Constant-time 38 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> 39</dl></div> 40<div class="section"> 41<div class="titlepage"><div><div><h3 class="title"> 42<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 43 initialization for vector-like containers</a> 44</h3></div></div></div> 45<p> 46 STL and most other containers value initialize new elements in common operations 47 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> 48 <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>. 49 </p> 50<p> 51 In some performance-sensitive environments, where vectors are used as a replacement 52 for variable-size buffers for file or network operations, <a href="http://en.cppreference.com/w/cpp/language/value_initialization" target="_top">value 53 initialization</a> is a cost that is not negligible as elements are going 54 to be overwritten by an external source shortly after new elements are added 55 to the container. 56 </p> 57<p> 58 <span class="bold"><strong>Boost.Container</strong></span> offers two new members for 59 <code class="computeroutput"><span class="identifier">vector</span></code>, <code class="computeroutput"><span class="identifier">static_vector</span></code> 60 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 61 <a href="http://en.cppreference.com/w/cpp/language/default_initialization" target="_top">default 62 initialization</a>. 63 </p> 64</div> 65<div class="section"> 66<div class="titlepage"><div><div><h3 class="title"> 67<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 68 range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>, 69 <span class="emphasis"><em>ordered_range</em></span>) </a> 70</h3></div></div></div> 71<p> 72 When filling associative containers big performance gains can be achieved 73 if the input range to be inserted is guaranteed by the user to be ordered 74 according to the predicate. This can happen when inserting values from a 75 <code class="computeroutput"><span class="identifier">set</span></code> to a <code class="computeroutput"><span class="identifier">multiset</span></code> 76 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> 77 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>). 78 </p> 79<p> 80 <span class="bold"><strong>Boost.Container</strong></span> has some overloads for constructors 81 and insertions taking an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code> 82 or an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> tag 83 parameters as the first argument. When an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code> 84 overload is used, the user notifies the container that the input range is 85 ordered according to the container predicate and has no duplicates. When 86 an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> overload 87 is used, the user notifies the container that the input range is ordered 88 according to the container predicate but it might have duplicates. With this 89 information, the container can avoid multiple predicate calls and improve 90 insertion times. 91 </p> 92</div> 93<div class="section"> 94<div class="titlepage"><div><div><h3 class="title"> 95<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 96 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> 97</h3></div></div></div> 98<p> 99 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 100 that caused some controversy in the C++ community. Quoting Howard Hinnant's 101 <a href="http://howardhinnant.github.io/On_list_size.html" target="_top"><span class="emphasis"><em>On 102 List Size</em></span></a> paper: 103 </p> 104<div class="blockquote"><blockquote class="blockquote"> 105<p> 106 <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 107 notes that it is a tradeoff with:</em></span> 108 </p> 109<p> 110 <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> 111 <span class="identifier">first</span><span class="special">,</span> 112 <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">);</span></code> 113 </p> 114<p> 115 <span class="emphasis"><em>If size() is O(1) and this != &x, then this method must perform 116 a linear operation so that it can adjust the size member in each list</em></span> 117 </p> 118</blockquote></div> 119<p> 120 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). 121 However, Howard Hinnant's paper proposed a new <code class="computeroutput"><span class="identifier">splice</span></code> 122 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> 123 implementations could achieve O(1) range splice when the range size was known 124 to the caller: 125 </p> 126<div class="blockquote"><blockquote class="blockquote"> 127<p> 128 <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> 129 <span class="identifier">first</span><span class="special">,</span> 130 <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">size_type</span> 131 <span class="identifier">n</span><span class="special">);</span></code> 132 </p> 133<p> 134 <span class="bold"><strong>Effects</strong></span>: Inserts elements in the range 135 [first, last) before position and removes the elements from x. 136 </p> 137<p> 138 <span class="bold"><strong>Requires</strong></span>: [first, last) is a valid range 139 in x. The result is undefined if position is an iterator in the range [first, 140 last). Invalidates only the iterators and references to the spliced elements. 141 n == distance(first, last). 142 </p> 143<p> 144 <span class="bold"><strong>Throws</strong></span>: Nothing. 145 </p> 146<p> 147 <span class="bold"><strong>Complexity</strong></span>: Constant time. 148 </p> 149</blockquote></div> 150<p> 151 This new splice signature allows the client to pass the distance of the input 152 range in. This information is often available at the call site. If it is 153 passed in, then the operation is constant time, even with an O(1) size. 154 </p> 155<p> 156 <span class="bold"><strong>Boost.Container</strong></span> implements this overload 157 for <code class="computeroutput"><span class="identifier">list</span></code> and a modified version 158 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> 159 is also <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>). 160 </p> 161</div> 162</div> 163<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 164<td align="left"></td> 165<td align="right"><div class="copyright-footer">Copyright © 2009-2018 Ion Gaztanaga<p> 166 Distributed under the Boost Software License, Version 1.0. (See accompanying 167 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>) 168 </p> 169</div></td> 170</tr></table> 171<hr> 172<div class="spirit-nav"> 173<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> 174</div> 175</body> 176</html> 177
