1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Implicit Move when returning a local object</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../move.html" title="Chapter 25. Boost.Move"> 10<link rel="prev" href="construct_forwarding.html" title="Constructor Forwarding"> 11<link rel="next" href="move_iterator.html" title="Move iterators"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="construct_forwarding.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_iterator.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="move.move_return"></a><a class="link" href="move_return.html" title="Implicit Move when returning a local object">Implicit Move when returning a local 29 object</a> 30</h2></div></div></div> 31<p> 32 The C++ standard specifies situations where an implicit move operation is safe 33 and the compiler can use it in cases were the (Named) Return Value Optimization) 34 can't be used. The typical use case is when a function returns a named (non-temporary) 35 object by value and the following code will perfectly compile in C++11: 36 </p> 37<pre class="programlisting"><span class="comment">//Even if movable can't be copied</span> 38<span class="comment">//the compiler will call the move-constructor</span> 39<span class="comment">//to generate the return value</span> 40<span class="comment">//</span> 41<span class="comment">//The compiler can also optimize out the move</span> 42<span class="comment">//and directly construct the object 'm'</span> 43<span class="identifier">movable</span> <span class="identifier">factory</span><span class="special">()</span> 44<span class="special">{</span> 45 <span class="identifier">movable</span> <span class="identifier">tmp</span><span class="special">;</span> 46 <span class="identifier">m</span> <span class="special">=</span> <span class="special">...</span> 47 <span class="comment">//(1) moved instead of copied</span> 48 <span class="keyword">return</span> <span class="identifier">tmp</span><span class="special">;</span> 49<span class="special">};</span> 50 51<span class="comment">//Initialize object</span> 52<span class="identifier">movable</span> <span class="identifier">m</span><span class="special">(</span><span class="identifier">factory</span><span class="special">());</span> 53</pre> 54<p> 55 In compilers without rvalue references and some non-conforming compilers (such 56 as Visual C++ 2010/2012) the line marked with <code class="computeroutput"><span class="special">(</span><span class="number">1</span><span class="special">)</span></code> would trigger 57 a compilation error because <code class="computeroutput"><span class="identifier">movable</span></code> 58 can't be copied. Using a explicit <code class="computeroutput"><span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">tmp</span><span class="special">)</span></code> 59 would workaround this limitation but it would code suboptimal in C++11 compilers 60 (as the compile could not use (N)RVO to optimize-away the copy/move). 61 </p> 62<p> 63 <span class="bold"><strong>Boost.Move</strong></span> offers an additional macro called 64 <code class="computeroutput"><a class="link" href="../BOOST_MOVE_RET.html" title="Macro BOOST_MOVE_RET">BOOST_MOVE_RET</a></code> that can be used 65 to alleviate this problem obtaining portable move-on-return semantics. Let's 66 use the previously presented movable-only <code class="computeroutput"><span class="identifier">movable</span></code> 67 class with classes <code class="computeroutput"><span class="identifier">copyable</span></code> 68 (copy-only type), <code class="computeroutput"><span class="identifier">copy_movable</span></code> 69 (can be copied and moved) and <code class="computeroutput"><span class="identifier">non_copy_movable</span></code> 70 (non-copyable and non-movable): 71 </p> 72<p> 73</p> 74<pre class="programlisting"><span class="comment">//header file "copymovable.hpp"</span> 75<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">move</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 76 77<span class="comment">//A copy_movable class</span> 78<span class="keyword">class</span> <span class="identifier">copy_movable</span> 79<span class="special">{</span> 80 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span> 81 <span class="keyword">int</span> <span class="identifier">value_</span><span class="special">;</span> 82 83 <span class="keyword">public</span><span class="special">:</span> 84 <span class="identifier">copy_movable</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="number">1</span><span class="special">){}</span> 85 86 <span class="comment">//Move constructor and assignment</span> 87 <span class="identifier">copy_movable</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span> 88 <span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> 89 90 <span class="identifier">copy_movable</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copy_movable</span> <span class="special">&</span><span class="identifier">m</span><span class="special">)</span> 91 <span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="special">}</span> 92 93 <span class="identifier">copy_movable</span> <span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span> 94 <span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 95 96 <span class="identifier">copy_movable</span> <span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">)</span> <span class="identifier">m</span><span class="special">)</span> 97 <span class="special">{</span> <span class="identifier">value_</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 98 99 <span class="keyword">bool</span> <span class="identifier">moved</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">//Observer</span> 100 <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">value_</span> <span class="special">==</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> 101<span class="special">};</span> 102 103<span class="comment">//A copyable-only class</span> 104<span class="keyword">class</span> <span class="identifier">copyable</span> 105<span class="special">{};</span> 106 107<span class="comment">//A copyable-only class</span> 108<span class="keyword">class</span> <span class="identifier">non_copy_movable</span> 109<span class="special">{</span> 110 <span class="keyword">public</span><span class="special">:</span> 111 <span class="identifier">non_copy_movable</span><span class="special">(){}</span> 112 <span class="keyword">private</span><span class="special">:</span> 113 <span class="identifier">non_copy_movable</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">non_copy_movable</span><span class="special">&);</span> 114 <span class="identifier">non_copy_movable</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">non_copy_movable</span><span class="special">&);</span> 115<span class="special">};</span> 116</pre> 117<p> 118 </p> 119<p> 120 and build a generic factory function that returns a newly constructed value 121 or a reference to an already constructed object. 122 </p> 123<p> 124</p> 125<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"movable.hpp"</span> 126<span class="preprocessor">#include</span> <span class="string">"copymovable.hpp"</span> 127<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">move</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 128 129<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Type</span><span class="special">></span> 130<span class="keyword">struct</span> <span class="identifier">factory_functor</span> 131<span class="special">{</span> 132 <span class="keyword">typedef</span> <span class="identifier">Type</span> <span class="identifier">return_type</span><span class="special">;</span> 133 134 <span class="identifier">Type</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> 135 <span class="special">{</span> <span class="identifier">Type</span> <span class="identifier">t</span><span class="special">;</span> <span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">Type</span><span class="special">,</span> <span class="identifier">t</span><span class="special">);</span> <span class="special">}</span> 136<span class="special">};</span> 137 138<span class="keyword">struct</span> <span class="identifier">return_reference</span> 139<span class="special">{</span> 140 <span class="keyword">typedef</span> <span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="identifier">return_type</span><span class="special">;</span> 141 142 <span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> 143 <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">ncm</span><span class="special">;</span> <span class="special">}</span> 144 145 <span class="keyword">static</span> <span class="identifier">non_copy_movable</span> <span class="identifier">ncm</span><span class="special">;</span> 146<span class="special">};</span> 147 148<span class="identifier">non_copy_movable</span> <span class="identifier">return_reference</span><span class="special">::</span><span class="identifier">ncm</span><span class="special">;</span> 149 150<span class="comment">//A wrapper that locks a mutex while the</span> 151<span class="comment">//factory creates a new value.</span> 152<span class="comment">//It must generically move the return value</span> 153<span class="comment">//if possible both in C++03 and C++11</span> 154<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Factory</span><span class="special">></span> 155<span class="keyword">typename</span> <span class="identifier">Factory</span><span class="special">::</span><span class="identifier">return_type</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">Factory</span> <span class="identifier">f</span><span class="special">)</span> 156<span class="special">{</span> 157 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Factory</span><span class="special">::</span><span class="identifier">return_type</span> <span class="identifier">return_type</span><span class="special">;</span> 158 <span class="comment">//LOCK();</span> 159 <span class="identifier">return_type</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">();</span> 160 <span class="comment">//UNLOCK();</span> 161 162 <span class="comment">//In C++03: boost::move() if R is not a reference and</span> 163 <span class="comment">//has move emulation enabled. In C++11: just return r.</span> 164 <span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">return_type</span><span class="special">,</span> <span class="identifier">r</span><span class="special">);</span> 165<span class="special">}</span> 166 167<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 168<span class="special">{</span> 169 <span class="identifier">movable</span> <span class="identifier">m</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">movable</span><span class="special">></span> <span class="special">());</span> 170 <span class="identifier">copy_movable</span> <span class="identifier">cm</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">copy_movable</span><span class="special">>());</span> 171 <span class="identifier">copyable</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">factory_functor</span><span class="special"><</span><span class="identifier">copyable</span><span class="special">></span> <span class="special">());</span> 172 <span class="identifier">non_copy_movable</span> <span class="special">&</span><span class="identifier">mr</span> <span class="special">=</span> <span class="identifier">lock_wrapper</span><span class="special">(</span><span class="identifier">return_reference</span> <span class="special">());</span> 173 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 174<span class="special">}</span> 175</pre> 176<p> 177 </p> 178<p> 179 <span class="bold"><strong>Caution</strong></span>: When using this macro in a non-conforming 180 or C++03 compilers, a move will be performed even if the C++11 standard does 181 not allow it (e.g. returning a static variable). The user is responsible for 182 using this macro only used to return local objects that met C++11 criteria. 183 E.g.: 184 </p> 185<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">foo</span> 186<span class="special">{</span> 187 <span class="identifier">copy_movable</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> 188 <span class="special">{</span> 189 <span class="comment">//ERROR! The Standard does not allow implicit move returns when the object to be returned </span> 190 <span class="comment">//does not met the criteria for elision of a copy operation (such as returning a static member data)</span> 191 <span class="comment">//In C++03 compilers this will MOVE resources from cm</span> 192 <span class="comment">//In C++11 compilers this will COPY resources from cm</span> 193 <span class="comment">//so DON'T use use BOOST_MOVE_RET without care.</span> 194 <span class="keyword">return</span> <span class="identifier">BOOST_MOVE_RET</span><span class="special">(</span><span class="identifier">copy_movable</span><span class="special">,</span> <span class="identifier">cm</span><span class="special">);</span> 195 <span class="special">}</span> 196 197 <span class="keyword">static</span> <span class="identifier">copy_movable</span> <span class="identifier">cm</span><span class="special">;</span> 198<span class="special">};</span> 199</pre> 200<p> 201 <span class="bold"><strong>Note</strong></span>: When returning a temporary object <code class="computeroutput"><span class="identifier">BOOST_MOVE_REF</span></code> is not needed as copy ellision 202 rules will work on both C++03 and C++11 compilers. 203 </p> 204<pre class="programlisting"><span class="comment">//Note: no BOOST_MOVE_RET</span> 205<span class="identifier">movable</span> <span class="identifier">get_movable</span><span class="special">()</span> 206<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">movable</span><span class="special">();</span> <span class="special">}</span> 207 208<span class="identifier">copy_movable</span> <span class="identifier">get_copy_movable</span><span class="special">()</span> 209<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">copy_movable</span><span class="special">();</span> <span class="special">}</span> 210 211<span class="identifier">copyable</span> <span class="identifier">get_copyable</span><span class="special">()</span> 212<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">copyable</span><span class="special">();</span> <span class="special">}</span> 213</pre> 214</div> 215<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 216<td align="left"></td> 217<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 218 Distributed under the Boost Software License, Version 1.0. (See accompanying 219 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>) 220 </p> 221</div></td> 222</tr></table> 223<hr> 224<div class="spirit-nav"> 225<a accesskey="p" href="construct_forwarding.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_iterator.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 226</div> 227</body> 228</html> 229
