boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Movable but Non-Copyable Types</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="../move.html" title="Chapter 19. Boost.Move"> <link rel="prev" href="composition_inheritance.html" title="Composition or inheritance"> <link rel="next" href="move_and_containers.html" title="Containers and move semantics"> </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="composition_inheritance.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="move_and_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="move.movable_only_classes"></a><a class="link" href="movable_only_classes.html" title="Movable but Non-Copyable Types">Movable but Non-Copyable Types</a> </h2></div></div></div> Some types are not amenable to copy semantics but can still be made movable. For example: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> <code class="computeroutput">unique_ptr</code> (non-shared, non-copyable ownership) </li> <li class="listitem"> A type representing a thread of execution </li> <li class="listitem"> A type representing a file descriptor </li> </ul></div> By making such types movable (though still non-copyable) their utility is tremendously increased. Movable but non-copyable types can be returned by value from factory functions: <pre class="programlisting">file_descriptor create_file(/* ... */); //... file_descriptor data_file; //... data_file = create_file(/* ... */); // No copies! </pre> In the above example, the underlying file handle is passed from object to object, as long as the source <code class="computeroutput">file_descriptor</code> is a rvalue. At all times, there is still only one underlying file handle, and only one <code class="computeroutput">file_descriptor</code> owns it at a time. To write a movable but not copyable type in portable syntax, you need to follow these simple steps: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> Put the following macro in the private section: <code class="computeroutput"><a class="link" href="../BOOST_MOVABLE_BUT_NOT_COPYABLE.html" title="Macro BOOST_MOVABLE_BUT_NOT_COPYABLE">BOOST_MOVABLE_BUT_NOT_COPYABLE(classname)</a></code> </li> <li class="listitem"> Write a move constructor and a move assignment taking the parameter as <code class="computeroutput"><a class="link" href="../BOOST_RV_REF.html" title="Macro BOOST_RV_REF">BOOST_RV_REF(classname)</a></code> </li> </ul></div> Here's the definition of <code class="computeroutput">file descriptor</code> using portable syntax: <pre class="programlisting">#include <boost/move/utility_core.hpp> #include <stdexcept> class file_descriptor { int os_descr_; private: BOOST_MOVABLE_BUT_NOT_COPYABLE(file_descriptor) public: explicit file_descriptor(const char *filename) //Constructor : os_descr_(operating_system_open_file(filename)) { if(!os_descr_) throw std::runtime_error("file not found"); } ~file_descriptor() //Destructor { if(os_descr_) operating_system_close_file(os_descr_); } file_descriptor(BOOST_RV_REF(file_descriptor) x) // Move ctor : os_descr_(x.os_descr_) { x.os_descr_ = 0; } file_descriptor& operator=(BOOST_RV_REF(file_descriptor) x) // Move assign { if(os_descr_) operating_system_close_file(os_descr_); os_descr_ = x.os_descr_; x.os_descr_ = 0; return *this; } bool empty() const { return os_descr_ == 0; } }; </pre> </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 © 2008-2014 Ion Gaztanaga 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>) </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="composition_inheritance.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="move_and_containers.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>