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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Emulation limitations</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../move.html" title="Chapter 25. Boost.Move"> <link rel="prev" href="move_algorithms.html" title="Move algorithms"> <link rel="next" href="how_the_library_works.html" title="How the library works"> </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="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> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="move.emulation_limitations"></a><a class="link" href="emulation_limitations.html" title="Emulation limitations">Emulation limitations</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_base">Initializing base classes</a></span></dt> <dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.template_parameters">Template parameters for perfect forwarding</a></span></dt> <dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.emulation_limitations_binding">Binding of rvalue references to lvalues</a></span></dt> <dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.assignment_operator">Assignment operator in classes derived from or holding copyable and movable types</a></span></dt> <dt><span class="section"><a href="emulation_limitations.html#move.emulation_limitations.templated_assignment_operator">Templated assignment operator in copyable and movable types</a></span></dt> </dl></div> <p> Like any emulation effort, the library has some limitations users should take in care to achieve portable and efficient code when using the library with C++03 conformant compilers: </p> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <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 base classes</a> </h3></div></div></div> <p> When initializing base classes in move constructors, users must cast the reference to a base class reference before moving it or just use <code class="computeroutput"><span class="identifier">BOOST_MOVE_BASE</span></code>. Example: </p> <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> <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> <span class="comment">//...</span> </pre> <p> or </p> <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> <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> <span class="comment">//...</span> </pre> <p> If casting is not performed the emulation will not move construct the base 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>. Without the cast or <code class="computeroutput"><span class="identifier">BOOST_MOVE_BASE</span></code> we might obtain a compilation error (for non-copyable types) or a less-efficient move constructor (for copyable types): </p> <pre class="programlisting"><span class="comment">//If Derived is copyable, then Base is copy-constructed.</span> <span class="comment">//If not, a compilation error is issued</span> <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> <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> <span class="comment">//...</span> </pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <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 parameters for perfect forwarding</a> </h3></div></div></div> <p> The emulation can't deal with C++0x reference collapsing rules that allow perfect forwarding: </p> <pre class="programlisting"><span class="comment">//C++0x</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <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> <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> <span class="comment">//Wrong C++03 emulation</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <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> <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> </pre> <p> In C++03 emulation BOOST_RV_REF doesn't catch any const rlvalues. For more details on forwarding see <a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor Forwarding</a> chapter. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <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 of rvalue references to lvalues</a> </h3></div></div></div> <p> The <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html" target="_top">first rvalue reference</a> proposal allowed the binding of rvalue references to lvalues: </p> <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> <span class="comment">//....</span> <span class="identifier">Type</span> <span class="identifier">t</span><span class="special">;</span> <span class="comment">//Allowed</span> <span class="identifier">func</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span> </pre> <p> 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 a Safety Problem with Rvalue References</em></span></a> this behaviour was considered dangerous and eliminated this binding so that rvalue references adhere to the principle of type-safe overloading: <span class="emphasis"><em>Every function must be type-safe in isolation, without regard to how it has been overloaded</em></span> </p> <p> <span class="bold"><strong>Boost.Move</strong></span> can't emulate this type-safe overloading principle for C++03 compilers: </p> <pre class="programlisting"><span class="comment">//Allowed by move emulation</span> <span class="identifier">movable</span> <span class="identifier">m</span><span class="special">;</span> <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> </pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <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 operator in classes derived from or holding copyable and movable types</a> </h3></div></div></div> <p> 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> needs to define a copy constructor for <code class="computeroutput"><span class="identifier">copyable_and_movable</span></code> taking a non-const parameter in C++03 compilers: </p> <pre class="programlisting"><span class="comment">//Generated by BOOST_COPYABLE_AND_MOVABLE</span> <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> </pre> <p> Since the non-const overload of the copy constructor is generated, compiler-generated assignment operators for classes containing <code class="computeroutput"><span class="identifier">copyable_and_movable</span></code> will get the non-const copy constructor overload, which will surely surprise users: </p> <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">holder</span> <span class="special">{</span> <span class="identifier">copyable_and_movable</span> <span class="identifier">c</span><span class="special">;</span> <span class="special">};</span> <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> <span class="special">{</span> <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> <span class="comment">//Compiler-generated copy constructor is non-const:</span> <span class="comment">// holder& operator(holder &)</span> <span class="comment">//!!!</span> <span class="special">}</span> </pre> <p> This limitation forces the user to define a const version of the copy assignment, in all classes holding copyable and movable classes which might be annoying in some cases. </p> <p> An alternative is to implement a single <code class="computeroutput"><span class="keyword">operator</span> <span class="special">=()</span></code> for copyable and movable classes <a href="http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/" target="_top">using "pass by value" semantics</a>: </p> <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> <span class="special">{</span> <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> <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> <span class="special">}</span> </pre> <p> However, "pass by value" is not optimal for classes (like containers, strings, etc.) that reuse resources (like previously allocated memory) when x is assigned from a lvalue. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <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 assignment operator in copyable and movable types</a> </h3></div></div></div> <p> Given a movable and copyable class, if a templated assignment operator (*) is added: </p> <p> </p> <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Foo</span> <span class="special">{</span> <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Foo</span><span class="special">)</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> <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> <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> <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> <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> <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> <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> <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> <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> <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> <span class="special">};</span> </pre> <p> </p> <p> 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> <span class="identifier">Foo</span></code> lvalue: </p> <p> </p> <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> <span class="identifier">Foo</span> <span class="identifier">foo2</span><span class="special">(</span><span class="number">2</span><span class="special">);</span> <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> <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> <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> </pre> <p> </p> <p> This different behaviour is a side-effect of the move emulation that can't be easily avoided by <span class="bold"><strong>Boost.Move</strong></span>. One workaround is to SFINAE-out the templated assignment operator with <code class="computeroutput"><span class="identifier">disable_if</span></code>: </p> <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> <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> <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> <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> </pre> </div> </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<p> 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>