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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>C++11 Compliance</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../unordered.html" title="Chapter 37. Boost.Unordered"> <link rel="prev" href="comparison.html" title="Comparison with Associative Containers"> <link rel="next" href="rationale.html" title="Implementation Rationale"> </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="comparison.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.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="rationale.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="unordered.compliance"></a><a class="link" href="compliance.html" title="C++11 Compliance">C++11 Compliance</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><span class="section"><a href="compliance.html#unordered.compliance.move">Move emulation</a></span></dt> <dt><span class="section"><a href="compliance.html#unordered.compliance.allocator_compliance">Use of allocators</a></span></dt> <dt><span class="section"><a href="compliance.html#unordered.compliance.pairs">Pairs</a></span></dt> <dt><span class="section"><a href="compliance.html#unordered.compliance.misc">Miscellaneous</a></span></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="unordered.compliance.move"></a><a class="link" href="compliance.html#unordered.compliance.move" title="Move emulation">Move emulation</a> </h3></div></div></div> <p> Support for move semantics is implemented using Boost.Move. If rvalue references are available it will use them, but if not it uses a close, but imperfect emulation. On such compilers: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> Non-copyable objects can be stored in the containers. They can be constructed in place using <code class="computeroutput"><span class="identifier">emplace</span></code>, or if they support Boost.Move, moved into place. </li> <li class="listitem"> The containers themselves are not movable. </li> <li class="listitem"> Argument forwarding is not perfect. </li> </ul></div> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="unordered.compliance.allocator_compliance"></a><a class="link" href="compliance.html#unordered.compliance.allocator_compliance" title="Use of allocators">Use of allocators</a> </h3></div></div></div> <p> C++11 introduced a new allocator system. It's backwards compatible due to the lax requirements for allocators in the old standard, but might need some changes for allocators which worked with the old versions of the unordered containers. It uses a traits class, <code class="computeroutput"><span class="identifier">allocator_traits</span></code> to handle the allocator adding extra functionality, and making some methods and types optional. During development a stable release of <code class="computeroutput"><span class="identifier">allocator_traits</span></code> wasn't available so an internal partial implementation is always used in this version. Hopefully a future version will use the standard implementation where available. </p> <p> The member functions <code class="computeroutput"><span class="identifier">construct</span></code>, <code class="computeroutput"><span class="identifier">destroy</span></code> and <code class="computeroutput"><span class="identifier">max_size</span></code> are now optional, if they're not available a fallback is used. A full implementation of <code class="computeroutput"><span class="identifier">allocator_traits</span></code> requires sophisticated member function detection so that the fallback is used whenever the member function call is not well formed. This requires support for SFINAE expressions, which are available on GCC from version 4.4 and Clang. </p> <p> On other compilers, there's just a test to see if the allocator has a member, but no check that it can be called. So rather than using a fallback there will just be a compile error. </p> <p> <code class="computeroutput"><span class="identifier">propagate_on_container_copy_assignment</span></code>, <code class="computeroutput"><span class="identifier">propagate_on_container_move_assignment</span></code>, <code class="computeroutput"><span class="identifier">propagate_on_container_swap</span></code> and <code class="computeroutput"><span class="identifier">select_on_container_copy_construction</span></code> are also supported. Due to imperfect move emulation, some assignments might check <code class="computeroutput"><span class="identifier">propagate_on_container_copy_assignment</span></code> on some compilers and <code class="computeroutput"><span class="identifier">propagate_on_container_move_assignment</span></code> on others. </p> <p> The use of the allocator's construct and destruct methods might be a bit surprising. Nodes are constructed and destructed using the allocator, but the elements are stored in aligned space within the node and constructed and destructed by calling the constructor and destructor directly. </p> <p> In C++11 the allocator's construct function has the signature: </p> <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">construct</span><span class="special">(</span><span class="identifier">U</span><span class="special">*</span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span> </pre> <p> which supports calling <code class="computeroutput"><span class="identifier">construct</span></code> for the contained object, but most existing allocators don't support this. If member function detection was good enough then with old allocators it would fall back to calling the element's constructor directly but in general, detection isn't good enough to do this which is why Boost.Unordered just calls the constructor directly every time. In most cases this will work okay. </p> <p> <code class="computeroutput"><span class="identifier">pointer_traits</span></code> aren't used. Instead, pointer types are obtained from rebound allocators, this can cause problems if the allocator can't be used with incomplete types. If <code class="computeroutput"><span class="identifier">const_pointer</span></code> is not defined in the allocator, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">pointer_to_other</span><span class="special"><</span><span class="identifier">pointer</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">value_type</span><span class="special">>::</span><span class="identifier">type</span></code> is used to obtain a const pointer. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="unordered.compliance.pairs"></a><a class="link" href="compliance.html#unordered.compliance.pairs" title="Pairs">Pairs</a> </h3></div></div></div> <p> Since the containers use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code> they're limited to the version from the current standard library. But since C++11 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>'s <code class="computeroutput"><span class="identifier">piecewise_construct</span></code> based constructor is very useful, <code class="computeroutput"><span class="identifier">emplace</span></code> emulates it with a <code class="computeroutput"><span class="identifier">piecewise_construct</span></code> in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered</span></code> namespace. So for example, the following will work: </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multimap</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">></span> <span class="identifier">x</span><span class="special">;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered</span><span class="special">::</span><span class="identifier">piecewise_construct</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="string">"key"</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">));</span> </pre> <p> Older drafts of the standard also supported variadic constructors for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>, where the first argument would be used for the first part of the pair, and the remaining for the second part. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="unordered.compliance.misc"></a><a class="link" href="compliance.html#unordered.compliance.misc" title="Miscellaneous">Miscellaneous</a> </h3></div></div></div> <p> When swapping, <code class="computeroutput"><span class="identifier">Pred</span></code> and <code class="computeroutput"><span class="identifier">Hash</span></code> are not currently swapped by calling <code class="computeroutput"><span class="identifier">swap</span></code>, their copy constructors are used. As a consequence when swapping an exception may be throw from their copy constructor. </p> <p> Variadic constructor arguments for <code class="computeroutput"><span class="identifier">emplace</span></code> are only used when both rvalue references and variadic template parameters are available. Otherwise <code class="computeroutput"><span class="identifier">emplace</span></code> can only take up to 10 constructors arguments. </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 © 2003, 2004 Jeremy B. Maitin-Shepard<br>Copyright © 2005-2008 Daniel James<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="comparison.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../unordered.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="rationale.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>