boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Composition or inheritance</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="implementing_movable_classes.html" title="Implementing copyable and movable classes"> <link rel="next" href="movable_only_classes.html" title="Movable but Non-Copyable Types"> </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="implementing_movable_classes.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="movable_only_classes.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.composition_inheritance"></a><a class="link" href="composition_inheritance.html" title="Composition or inheritance">Composition or inheritance</a> </h2></div></div></div> For classes made up of other classes (via either composition or inheritance), the move constructor and move assignment can be easily coded using the <code class="computeroutput">boost::move</code> function: <pre class="programlisting">class Base { BOOST_COPYABLE_AND_MOVABLE(Base) public: Base(){} Base(const Base &/*x*/) {/**/} // Copy ctor Base(BOOST_RV_REF(Base) /*x*/) {/**/} // Move ctor Base& operator=(BOOST_RV_REF(Base) /*x*/) {/**/ return *this;} // Move assign Base& operator=(BOOST_COPY_ASSIGN_REF(Base) /*x*/) {/**/ return *this;} // Copy assign virtual Base *clone() const { return new Base(*this); } virtual ~Base(){} }; class Member { BOOST_COPYABLE_AND_MOVABLE(Member) public: Member(){} // Compiler-generated copy constructor... Member(BOOST_RV_REF(Member)) {/**/} // Move ctor Member &operator=(BOOST_RV_REF(Member)) // Move assign {/**/ return *this; } Member &operator=(BOOST_COPY_ASSIGN_REF(Member)) // Copy assign {/**/ return *this; } }; class Derived : public Base { BOOST_COPYABLE_AND_MOVABLE(Derived) Member mem_; public: Derived(){} // Compiler-generated copy constructor... Derived(BOOST_RV_REF(Derived) x) // Move ctor : Base(boost::move(static_cast<Base&>(x))), mem_(boost::move(x.mem_)) { } Derived& operator=(BOOST_RV_REF(Derived) x) // Move assign { Base::operator=(boost::move(static_cast<Base&>(x))); mem_ = boost::move(x.mem_); return *this; } Derived& operator=(BOOST_COPY_ASSIGN_REF(Derived) x) // Copy assign { Base::operator=(static_cast<const Base&>(x)); mem_ = x.mem_; return *this; } // ... }; </pre> <div class="important"><table border="0" summary="Important"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td> <th align="left">Important</th> </tr> <tr><td align="left" valign="top"> Due to limitations in the emulation code, a cast to <code class="computeroutput">Base &</code> is needed before moving the base part in the move constructor and call Base's move constructor instead of the copy constructor. </td></tr> </table></div> Each subobject will now be treated individually, calling move to bind to the subobject's move constructors and move assignment operators. <code class="computeroutput">Member</code> has move operations coded (just like our earlier <code class="computeroutput">clone_ptr</code> example) which will completely avoid the tremendously more expensive copy operations: <pre class="programlisting">Derived d; Derived d2(boost::move(d)); d2 = boost::move(d); </pre> Note above that the argument x is treated as a lvalue reference. That's why it is necessary to say <code class="computeroutput">move(x)</code> instead of just x when passing down to the base class. This is a key safety feature of move semantics designed to prevent accidently moving twice from some named variable. All moves from lvalues occur explicitly. </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="implementing_movable_classes.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="movable_only_classes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>