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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="move.construct_forwarding"></a><a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor Forwarding</a> |
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| <p> |
| Consider writing a generic factory function that returns an object for a newly |
| constructed generic type. Factory functions such as this are valuable for encapsulating |
| and localizing the allocation of resources. Obviously, the factory function |
| must accept exactly the same sets of arguments as the constructors of the type |
| of objects constructed: |
| </p> |
| <pre class="programlisting"><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="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">()</span> |
| <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">();</span> <span class="special">}</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="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> |
| <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> <span class="special">}</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="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">)</span> |
| <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">);</span> <span class="special">}</span> |
| </pre> |
| <p> |
| Unfortunately, in C++03 the much bigger issue with this approach is that the |
| N-argument case would require 2^N overloads, immediately discounting this as |
| a general solution. Fortunately, most constructors take arguments by value, |
| by const-reference or by rvalue reference. If these limitations are accepted, |
| the forwarding emulation of a N-argument case requires just N overloads. This |
| library makes this emulation easy with the help of <code class="computeroutput"><span class="identifier">BOOST_FWD_REF</span></code> |
| and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span></code>: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><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">utility_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| |
| <span class="keyword">class</span> <span class="identifier">copyable_only_tester</span> |
| <span class="special">{</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">copyable_only_tester</span><span class="special">()</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester()"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copyable_only_tester</span><span class="special">&)</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(const copyable_only_tester&)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(int)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(int, double)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">class</span> <span class="identifier">copyable_movable_tester</span> |
| <span class="special">{</span> |
| <span class="comment">// move semantics</span> |
| <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">)</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <span class="identifier">copyable_movable_tester</span><span class="special">()</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester()"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(int)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copyable_movable_tester</span> <span class="special">&)</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(const copyable_movable_tester &)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">),</span> <span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> |
| |
| <span class="identifier">copyable_movable_tester</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">copyable_movable_tester</span><span class="special">))</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester & operator=(BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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">copyable_movable_tester</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">copyable_movable_tester</span><span class="special">))</span> |
| <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester & operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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="special">};</span> |
| |
| <span class="comment">//1 argument</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">MaybeMovable</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">function_construct</span><span class="special">(</span><span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> |
| <span class="special">{</span> <span class="identifier">MaybeMovable</span> <span class="identifier">m</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">MaybeRv</span><span class="special">>(</span><span class="identifier">x</span><span class="special">));</span> <span class="special">}</span> |
| |
| <span class="comment">//2 argument</span> |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">MaybeMovable</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv2</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">function_construct</span><span class="special">(</span><span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv</span><span class="special">)</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv2</span><span class="special">)</span> <span class="identifier">x2</span><span class="special">)</span> |
| <span class="special">{</span> <span class="identifier">MaybeMovable</span> <span class="identifier">m</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">MaybeRv</span><span class="special">>(</span><span class="identifier">x</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">MaybeRv2</span><span class="special">>(</span><span class="identifier">x2</span><span class="special">));</span> <span class="special">}</span> |
| |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">copyable_movable_tester</span> <span class="identifier">m</span><span class="special">;</span> |
| <span class="comment">//move constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</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">m</span><span class="special">));</span> |
| <span class="comment">//copy constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</span><span class="special">>(</span><span class="identifier">copyable_movable_tester</span><span class="special">());</span> |
| <span class="comment">//two rvalue constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</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">m</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">m</span><span class="special">));</span> |
| |
| <span class="identifier">copyable_only_tester</span> <span class="identifier">nm</span><span class="special">;</span> |
| <span class="comment">//copy constructor (copyable_only_tester has no move ctor.)</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</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">nm</span><span class="special">));</span> |
| <span class="comment">//copy constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="identifier">nm</span><span class="special">);</span> |
| <span class="comment">//int constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">));</span> |
| <span class="comment">//int, double constructor</span> |
| <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="keyword">double</span><span class="special">(</span><span class="number">0.0</span><span class="special">));</span> |
| |
| <span class="comment">//Output is:</span> |
| <span class="comment">//copyable_movable_tester()</span> |
| <span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))</span> |
| <span class="comment">//copyable_movable_tester()</span> |
| <span class="comment">//copyable_movable_tester(const copyable_movable_tester &)</span> |
| <span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))</span> |
| <span class="comment">//copyable_only_tester()</span> |
| <span class="comment">//copyable_only_tester(const copyable_only_tester&)</span> |
| <span class="comment">//copyable_only_tester(const copyable_only_tester&)</span> |
| <span class="comment">//copyable_only_tester(int)</span> |
| <span class="comment">//copyable_only_tester(int, double)</span> |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Constructor forwarding comes in handy to implement placement insertion in containers |
| with just N overloads if the implementor accepts the limitations of this type |
| of forwarding for C++03 compilers. In compilers with rvalue references perfect |
| forwarding is achieved. |
| </p> |
| </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> |
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