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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Implementing copyable and movable classes</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="introduction.html" title="Introduction"> <link rel="next" href="composition_inheritance.html" title="Composition or inheritance"> </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="introduction.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="composition_inheritance.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.implementing_movable_classes"></a><a class="link" href="implementing_movable_classes.html" title="Implementing copyable and movable classes">Implementing copyable and movable classes</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp0x">Copyable and movable classes in C++0x</a></dt> <dt><a href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp03">Copyable and movable classes in portable syntax for both C++03 and C++0x compilers</a></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="move.implementing_movable_classes.copyable_and_movable_cpp0x"></a><a class="link" href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp0x" title="Copyable and movable classes in C++0x">Copyable and movable classes in C++0x</a> </h3></div></div></div> Consider a simple handle class that owns a resource and also provides copy semantics (copy constructor and assignment). For example a <code class="computeroutput">clone_ptr</code> might own a pointer, and call <code class="computeroutput">clone()</code> on it for copying purposes: <pre class="programlisting">template <class T> class clone_ptr { private: T* ptr; public: // construction explicit clone_ptr(T* p = 0) : ptr(p) {} // destruction ~clone_ptr() { delete ptr; } // copy semantics clone_ptr(const clone_ptr& p) : ptr(p.ptr ? p.ptr->clone() : 0) {} clone_ptr& operator=(const clone_ptr& p) { if (this != &p) { T *p = p.ptr ? p.ptr->clone() : 0; delete ptr; ptr = p; } return *this; } // move semantics clone_ptr(clone_ptr&& p) : ptr(p.ptr) { p.ptr = 0; } clone_ptr& operator=(clone_ptr&& p) { if(this != &p) { std::swap(ptr, p.ptr); delete p.ptr; p.ptr = 0; } return *this; } // Other operations... }; </pre> <code class="computeroutput">clone_ptr</code> has expected copy constructor and assignment semantics, duplicating resources when copying. Note that copy constructing or assigning a <code class="computeroutput">clone_ptr</code> is a relatively expensive operation: <pre class="programlisting">clone_ptr<Base> p1(new Derived()); // ... clone_ptr<Base> p2 = p1; // p2 and p1 each own their own pointer </pre> <code class="computeroutput">clone_ptr</code> is code that you might find in today's books on C++, except for the part marked as <code class="computeroutput">move semantics</code>. That part is implemented in terms of C++0x <code class="computeroutput">rvalue references</code>. You can find some good introduction and tutorials on rvalue references in these papers: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> <a href="http://www.artima.com/cppsource/rvalue.html" target="_top">A Brief Introduction to Rvalue References</a> </li> <li class="listitem"> <a href="http://blogs.msdn.com/vcblog/archive/2009/02/03/rvalue-references-c-0x-features-in-vc10-part-2.aspx" target="_top">Rvalue References: C++0x Features in VC10, Part 2</a> </li> </ul></div> When the source of the copy is known to be a <code class="computeroutput">rvalue</code> (e.g.: a temporary object), one can avoid the potentially expensive <code class="computeroutput">clone()</code> operation by pilfering source's pointer (no one will notice!). The move constructor above does exactly that, leaving the rvalue in a default constructed state. The move assignment operator simply does the same freeing old resources. Now when code tries to copy a rvalue <code class="computeroutput">clone_ptr</code>, or if that code explicitly gives permission to consider the source of the copy a rvalue (using <code class="computeroutput">boost::move</code>), the operation will execute much faster. <pre class="programlisting">clone_ptr<Base> p1(new Derived()); // ... clone_ptr<Base> p2 = boost::move(p1); // p2 now owns the pointer instead of p1 p2 = clone_ptr<Base>(new Derived()); // temporary is moved to p2 } </pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="move.implementing_movable_classes.copyable_and_movable_cpp03"></a><a class="link" href="implementing_movable_classes.html#move.implementing_movable_classes.copyable_and_movable_cpp03" title="Copyable and movable classes in portable syntax for both C++03 and C++0x compilers">Copyable and movable classes in portable syntax for both C++03 and C++0x compilers</a> </h3></div></div></div> Many aspects of move semantics can be emulated for compilers not supporting <code class="computeroutput">rvalue references</code> and Boost.Move offers tools for that purpose. With Boost.Move we can write <code class="computeroutput">clone_ptr</code> so that it will work both in compilers with rvalue references and those who conform to C++03. You just 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_COPYABLE_AND_MOVABLE.html" title="Macro BOOST_COPYABLE_AND_MOVABLE">BOOST_COPYABLE_AND_MOVABLE(classname)</a></code> </li> <li class="listitem"> Leave copy constructor as is. </li> <li class="listitem"> Write a copy assignment taking the parameter as <code class="computeroutput"><a class="link" href="../BOOST_COPY_ASSIGN_REF.html" title="Macro BOOST_COPY_ASSIGN_REF">BOOST_COPY_ASSIGN_REF(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> Let's see how are applied to <code class="computeroutput">clone_ptr</code>: <pre class="programlisting">template <class T> class clone_ptr { private: // Mark this class copyable and movable BOOST_COPYABLE_AND_MOVABLE(clone_ptr) T* ptr; public: // Construction explicit clone_ptr(T* p = 0) : ptr(p) {} // Destruction ~clone_ptr() { delete ptr; } clone_ptr(const clone_ptr& p) // Copy constructor (as usual) : ptr(p.ptr ? p.ptr->clone() : 0) {} clone_ptr& operator=(BOOST_COPY_ASSIGN_REF(clone_ptr) p) // Copy assignment { if (this != &p){ T *tmp_p = p.ptr ? p.ptr->clone() : 0; delete ptr; ptr = tmp_p; } return *this; } //Move semantics... clone_ptr(BOOST_RV_REF(clone_ptr) p) //Move constructor : ptr(p.ptr) { p.ptr = 0; } clone_ptr& operator=(BOOST_RV_REF(clone_ptr) p) //Move assignment { if (this != &p){ delete ptr; ptr = p.ptr; p.ptr = 0; } return *this; } }; </pre> </div> Question: What about types that don't own resources? (E.g. <code class="computeroutput">std::complex</code>?) No work needs to be done in that case. The copy constructor is already optimal. </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="introduction.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="composition_inheritance.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>