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<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>
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<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>
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