diff options
Diffstat (limited to 'doc/html/move/emulation_limitations.html')
| -rwxr-xr-x | doc/html/move/emulation_limitations.html | 197 |
1 files changed, 197 insertions, 0 deletions
diff --git a/doc/html/move/emulation_limitations.html b/doc/html/move/emulation_limitations.html new file mode 100755 index 0000000000..00b0378dda --- /dev/null +++ b/doc/html/move/emulation_limitations.html @@ -0,0 +1,197 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Emulation limitations</title> +<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> +<link rel="up" href="../move.html" title="Chapter 16. 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> +<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> +</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. Example: + </p> +<pre class="programlisting"><span class="comment">//Portable and efficient</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="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="identifier">mem_</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="identifier">mem_</span><span class="special">))</span> <span class="special">{</span> <span class="special">}</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 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="identifier">mem_</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="identifier">mem_</span><span class="special">))</span> <span class="special">{</span> <span class="special">}</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 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> +<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 © 2008-2010 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> |