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>Emulation limitations</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="move_algorithms.html" title="Move algorithms"> 11<link rel="next" href="how_the_library_works.html" title="How the library works"> 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="move_algorithms.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="how_the_library_works.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.emulation_limitations"></a><a class="link" href="emulation_limitations.html" title="Emulation limitations">Emulation limitations</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_base">Initializing 32 base classes</a></span></dt> 33<dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.template_parameters">Template 34 parameters for perfect forwarding</a></span></dt> 35<dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_binding">Binding 36 of rvalue references to lvalues</a></span></dt> 37<dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.assignment_operator">Assignment 38 operator in classes derived from or holding copyable and movable types</a></span></dt> 39<dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.templated_assignment_operator">Templated 40 assignment operator in copyable and movable types</a></span></dt> 41</dl></div> 42<p> 43 Like any emulation effort, the library has some limitations users should take 44 in care to achieve portable and efficient code when using the library with 45 C++03 conformant compilers: 46 </p> 47<div class="section"> 48<div class="titlepage"><div><div><h3 class="title"> 49<a name="move.emulation_limitations.emulation_limitations_base"></a><a class="link" href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_base" title="Initializing base classes">Initializing 50 base classes</a> 51</h3></div></div></div> 52<p> 53 When initializing base classes in move constructors, users must cast the 54 reference to a base class reference before moving it or just use <code class="computeroutput"><span class="identifier">BOOST_MOVE_BASE</span></code>. Example: 55 </p> 56<pre class="programlisting"><span class="identifier">Derived</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move ctor</span> 57 <span class="special">:</span> <span class="identifier">Base</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="keyword">static_cast</span><span class="special"><</span><span class="identifier">Base</span><span class="special">&>(</span><span class="identifier">x</span><span class="special">)))</span> 58 <span class="comment">//...</span> 59</pre> 60<p> 61 or 62 </p> 63<pre class="programlisting"><span class="identifier">Derived</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move ctor</span> 64 <span class="special">:</span> <span class="identifier">Base</span><span class="special">(</span><span class="identifier">BOOST_MOVE_BASE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">x</span><span class="special">))</span> 65 <span class="comment">//...</span> 66</pre> 67<p> 68 If casting is not performed the emulation will not move construct the base 69 class, because no conversion is available from <code class="computeroutput"><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span></code> to <code class="computeroutput"><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span></code>. 70 Without the cast or <code class="computeroutput"><span class="identifier">BOOST_MOVE_BASE</span></code> 71 we might obtain a compilation error (for non-copyable types) or a less-efficient 72 move constructor (for copyable types): 73 </p> 74<pre class="programlisting"><span class="comment">//If Derived is copyable, then Base is copy-constructed.</span> 75<span class="comment">//If not, a compilation error is issued</span> 76<span class="identifier">Derived</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// Move ctor</span> 77 <span class="special">:</span> <span class="identifier">Base</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">x</span><span class="special">))</span> 78 <span class="comment">//...</span> 79</pre> 80</div> 81<div class="section"> 82<div class="titlepage"><div><div><h3 class="title"> 83<a name="move.emulation_limitations.template_parameters"></a><a class="link" href="emulation_limitations.html#move.emulation_limitations.template_parameters" title="Template parameters for perfect forwarding">Template 84 parameters for perfect forwarding</a> 85</h3></div></div></div> 86<p> 87 The emulation can't deal with C++0x reference collapsing rules that allow 88 perfect forwarding: 89 </p> 90<pre class="programlisting"><span class="comment">//C++0x</span> 91<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 92<span class="keyword">void</span> <span class="identifier">forward_function</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&&</span><span class="identifier">t</span><span class="special">)</span> 93<span class="special">{</span> <span class="identifier">inner_function</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">t</span><span class="special">);</span> <span class="special">}</span> 94 95<span class="comment">//Wrong C++03 emulation</span> 96<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 97<span class="keyword">void</span> <span class="identifier">forward_function</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">t</span><span class="special">)</span> 98<span class="special">{</span> <span class="identifier">inner_function</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">t</span><span class="special">);</span> <span class="special">}</span> 99</pre> 100<p> 101 In C++03 emulation BOOST_RV_REF doesn't catch any const rlvalues. For more 102 details on forwarding see <a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor 103 Forwarding</a> chapter. 104 </p> 105</div> 106<div class="section"> 107<div class="titlepage"><div><div><h3 class="title"> 108<a name="move.emulation_limitations.emulation_limitations_binding"></a><a class="link" href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_binding" title="Binding of rvalue references to lvalues">Binding 109 of rvalue references to lvalues</a> 110</h3></div></div></div> 111<p> 112 The <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html" target="_top">first 113 rvalue reference</a> proposal allowed the binding of rvalue references 114 to lvalues: 115 </p> 116<pre class="programlisting"><span class="identifier">func</span><span class="special">(</span><span class="identifier">Type</span> <span class="special">&&</span><span class="identifier">t</span><span class="special">);</span> 117<span class="comment">//....</span> 118 119<span class="identifier">Type</span> <span class="identifier">t</span><span class="special">;</span> <span class="comment">//Allowed</span> 120<span class="identifier">func</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span> 121</pre> 122<p> 123 Later, as explained in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2008/n2812.html" target="_top"><span class="emphasis"><em>Fixing 124 a Safety Problem with Rvalue References</em></span></a> this behaviour 125 was considered dangerous and eliminated this binding so that rvalue references 126 adhere to the principle of type-safe overloading: <span class="emphasis"><em>Every function 127 must be type-safe in isolation, without regard to how it has been overloaded</em></span> 128 </p> 129<p> 130 <span class="bold"><strong>Boost.Move</strong></span> can't emulate this type-safe 131 overloading principle for C++03 compilers: 132 </p> 133<pre class="programlisting"><span class="comment">//Allowed by move emulation</span> 134<span class="identifier">movable</span> <span class="identifier">m</span><span class="special">;</span> 135<span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">movable</span><span class="special">)</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">m</span><span class="special">;</span> 136</pre> 137</div> 138<div class="section"> 139<div class="titlepage"><div><div><h3 class="title"> 140<a name="move.emulation_limitations.assignment_operator"></a><a class="link" href="emulation_limitations.html#move.emulation_limitations.assignment_operator" title="Assignment operator in classes derived from or holding copyable and movable types">Assignment 141 operator in classes derived from or holding copyable and movable types</a> 142</h3></div></div></div> 143<p> 144 The macro <code class="computeroutput"><a class="link" href="../BOOST_COPYABLE_AND_MOVABLE.html" title="Macro BOOST_COPYABLE_AND_MOVABLE">BOOST_COPYABLE_AND_MOVABLE</a></code> 145 needs to define a copy constructor for <code class="computeroutput"><span class="identifier">copyable_and_movable</span></code> 146 taking a non-const parameter in C++03 compilers: 147 </p> 148<pre class="programlisting"><span class="comment">//Generated by BOOST_COPYABLE_AND_MOVABLE</span> 149<span class="identifier">copyable_and_movable</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">copyable_and_movable</span><span class="special">&){/**/}</span> 150</pre> 151<p> 152 Since the non-const overload of the copy constructor is generated, compiler-generated 153 assignment operators for classes containing <code class="computeroutput"><span class="identifier">copyable_and_movable</span></code> 154 will get the non-const copy constructor overload, which will surely surprise 155 users: 156 </p> 157<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">holder</span> 158<span class="special">{</span> 159 <span class="identifier">copyable_and_movable</span> <span class="identifier">c</span><span class="special">;</span> 160<span class="special">};</span> 161 162<span class="keyword">void</span> <span class="identifier">func</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">holder</span><span class="special">&</span> <span class="identifier">h</span><span class="special">)</span> 163<span class="special">{</span> 164 <span class="identifier">holder</span> <span class="identifier">copy_h</span><span class="special">(</span><span class="identifier">h</span><span class="special">);</span> <span class="comment">//<--- ERROR: can't convert 'const holder&' to 'holder&'</span> 165 <span class="comment">//Compiler-generated copy constructor is non-const:</span> 166 <span class="comment">// holder& operator(holder &)</span> 167 <span class="comment">//!!!</span> 168<span class="special">}</span> 169</pre> 170<p> 171 This limitation forces the user to define a const version of the copy assignment, 172 in all classes holding copyable and movable classes which might be annoying 173 in some cases. 174 </p> 175<p> 176 An alternative is to implement a single <code class="computeroutput"><span class="keyword">operator</span> 177 <span class="special">=()</span></code> for copyable and movable classes 178 <a href="http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/" target="_top">using 179 "pass by value" semantics</a>: 180 </p> 181<pre class="programlisting"><span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="comment">// x is a copy of the source; hard work already done</span> 182<span class="special">{</span> 183 <span class="identifier">swap</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">x</span><span class="special">);</span> <span class="comment">// trade our resources for x's</span> 184 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="comment">// our (old) resources get destroyed with x</span> 185<span class="special">}</span> 186</pre> 187<p> 188 However, "pass by value" is not optimal for classes (like containers, 189 strings, etc.) that reuse resources (like previously allocated memory) when 190 x is assigned from a lvalue. 191 </p> 192</div> 193<div class="section"> 194<div class="titlepage"><div><div><h3 class="title"> 195<a name="move.emulation_limitations.templated_assignment_operator"></a><a class="link" href="emulation_limitations.html#move.emulation_limitations.templated_assignment_operator" title="Templated assignment operator in copyable and movable types">Templated 196 assignment operator in copyable and movable types</a> 197</h3></div></div></div> 198<p> 199 Given a movable and copyable class, if a templated assignment operator (*) 200 is added: 201 </p> 202<p> 203</p> 204<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Foo</span> 205<span class="special">{</span> 206 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Foo</span><span class="special">)</span> 207 208 <span class="keyword">public</span><span class="special">:</span> 209 <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 210 <span class="keyword">explicit</span> <span class="identifier">Foo</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">val</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">i</span><span class="special">(</span><span class="identifier">val</span><span class="special">)</span> <span class="special">{}</span> 211 212 <span class="identifier">Foo</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Foo</span><span class="special">)</span> <span class="identifier">obj</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">i</span><span class="special">(</span><span class="identifier">obj</span><span class="special">.</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{}</span> 213 214 <span class="identifier">Foo</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">Foo</span><span class="special">)</span> <span class="identifier">rhs</span><span class="special">)</span> 215 <span class="special">{</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">i</span><span class="special">;</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">i</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> 216 217 <span class="identifier">Foo</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">Foo</span><span class="special">)</span> <span class="identifier">rhs</span><span class="special">)</span> 218 <span class="special">{</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">i</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> <span class="comment">//(1)</span> 219 220 <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="comment">//(*) TEMPLATED ASSIGNMENT, potential problem</span> 221 <span class="identifier">Foo</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">rhs</span><span class="special">)</span> 222 <span class="special">{</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">i</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> <span class="comment">//(2)</span> 223<span class="special">};</span> 224</pre> 225<p> 226 </p> 227<p> 228 C++98 and C++11 compilers will behave different when assigning from a <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> 229 <span class="identifier">Foo</span></code> lvalue: 230 </p> 231<p> 232</p> 233<pre class="programlisting"><span class="identifier">Foo</span> <span class="identifier">foo1</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> 234<span class="identifier">Foo</span> <span class="identifier">foo2</span><span class="special">(</span><span class="number">2</span><span class="special">);</span> 235<span class="identifier">foo2</span> <span class="special">=</span> <span class="identifier">foo1</span><span class="special">;</span> <span class="comment">// Calls (1) in C++11 but (2) in C++98</span> 236<span class="keyword">const</span> <span class="identifier">Foo</span> <span class="identifier">foo5</span><span class="special">(</span><span class="number">5</span><span class="special">);</span> 237<span class="identifier">foo2</span> <span class="special">=</span> <span class="identifier">foo5</span><span class="special">;</span> <span class="comment">// Calls (1) in C++11 but (2) in C++98</span> 238</pre> 239<p> 240 </p> 241<p> 242 This different behaviour is a side-effect of the move emulation that can't 243 be easily avoided by <span class="bold"><strong>Boost.Move</strong></span>. One workaround 244 is to SFINAE-out the templated assignment operator with <code class="computeroutput"><span class="identifier">disable_if</span></code>: 245 </p> 246<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="comment">// Modified templated assignment</span> 247<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">U</span><span class="special">,</span> <span class="identifier">Foo</span><span class="special">>,</span> <span class="identifier">Foo</span><span class="special">&>::</span><span class="identifier">type</span> 248 <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">rhs</span><span class="special">)</span> 249<span class="special">{</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">i</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> <span class="comment">//(2)</span> 250</pre> 251</div> 252</div> 253<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 254<td align="left"></td> 255<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 256 Distributed under the Boost Software License, Version 1.0. (See accompanying 257 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>) 258 </p> 259</div></td> 260</tr></table> 261<hr> 262<div class="spirit-nav"> 263<a accesskey="p" href="move_algorithms.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="how_the_library_works.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 264</div> 265</body> 266</html> 267
