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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="container.extended_functionality"></a><a class="link" href="extended_functionality.html" title="Extended functionality: Basic extensions">Extended functionality:
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Basic extensions</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.default_initialialization">Default
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initialization for vector-like containers</a></span></dt>
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<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.ordered_range_insertion">Ordered
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range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>,
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<span class="emphasis"><em>ordered_range</em></span>) </a></span></dt>
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<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.constant_time_range_splice">Constant-time
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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>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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
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initialization for vector-like containers</a>
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</h3></div></div></div>
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<p>
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STL and most other containers value initialize new elements in common operations
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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>
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<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>.
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</p>
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<p>
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In some performance-sensitive environments, where vectors are used as a replacement
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for variable-size buffers for file or network operations, <a href="http://en.cppreference.com/w/cpp/language/value_initialization" target="_top">value
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initialization</a> is a cost that is not negligible as elements are going
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to be overwritten by an external source shortly after new elements are added
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to the container.
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</p>
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<p>
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<span class="bold"><strong>Boost.Container</strong></span> offers two new members for
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<code class="computeroutput"><span class="identifier">vector</span></code>, <code class="computeroutput"><span class="identifier">static_vector</span></code>
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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
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<a href="http://en.cppreference.com/w/cpp/language/default_initialization" target="_top">default
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initialization</a>.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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
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range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>,
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<span class="emphasis"><em>ordered_range</em></span>) </a>
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</h3></div></div></div>
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<p>
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When filling associative containers big performance gains can be achieved
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if the input range to be inserted is guaranteed by the user to be ordered
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according to the predicate. This can happen when inserting values from a
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<code class="computeroutput"><span class="identifier">set</span></code> to a <code class="computeroutput"><span class="identifier">multiset</span></code>
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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>
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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>).
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</p>
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<p>
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<span class="bold"><strong>Boost.Container</strong></span> has some overloads for constructors
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and insertions taking an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code>
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or an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> tag
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parameters as the first argument. When an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code>
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overload is used, the user notifies the container that the input range is
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ordered according to the container predicate and has no duplicates. When
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an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> overload
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is used, the user notifies the container that the input range is ordered
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according to the container predicate but it might have duplicates. With this
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information, the container can avoid multiple predicate calls and improve
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insertion times.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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
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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>
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</h3></div></div></div>
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<p>
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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
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that caused some controversy in the C++ community. Quoting Howard Hinnant's
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<a href="http://howardhinnant.github.io/On_list_size.html" target="_top"><span class="emphasis"><em>On
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List Size</em></span></a> paper:
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</p>
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<div class="blockquote"><blockquote class="blockquote">
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<p>
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<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
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notes that it is a tradeoff with:</em></span>
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</p>
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<p>
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<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>
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<span class="identifier">first</span><span class="special">,</span>
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<span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">);</span></code>
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</p>
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<p>
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<span class="emphasis"><em>If size() is O(1) and this != &x, then this method must perform
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a linear operation so that it can adjust the size member in each list</em></span>
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</p>
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</blockquote></div>
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<p>
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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).
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However, Howard Hinnant's paper proposed a new <code class="computeroutput"><span class="identifier">splice</span></code>
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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>
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implementations could achieve O(1) range splice when the range size was known
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to the caller:
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</p>
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<div class="blockquote"><blockquote class="blockquote">
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<p>
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<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>
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<span class="identifier">first</span><span class="special">,</span>
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<span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">size_type</span>
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<span class="identifier">n</span><span class="special">);</span></code>
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</p>
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<p>
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<span class="bold"><strong>Effects</strong></span>: Inserts elements in the range
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[first, last) before position and removes the elements from x.
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</p>
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<p>
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<span class="bold"><strong>Requires</strong></span>: [first, last) is a valid range
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in x. The result is undefined if position is an iterator in the range [first,
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last). Invalidates only the iterators and references to the spliced elements.
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n == distance(first, last).
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</p>
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<p>
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<span class="bold"><strong>Throws</strong></span>: Nothing.
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</p>
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<p>
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<span class="bold"><strong>Complexity</strong></span>: Constant time.
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</p>
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</blockquote></div>
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<p>
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This new splice signature allows the client to pass the distance of the input
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range in. This information is often available at the call site. If it is
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passed in, then the operation is constant time, even with an O(1) size.
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</p>
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<p>
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<span class="bold"><strong>Boost.Container</strong></span> implements this overload
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for <code class="computeroutput"><span class="identifier">list</span></code> and a modified version
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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>
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is also <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>).
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</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2009-2018 Ion Gaztanaga<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div></td>
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</tr></table>
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