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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="move.construct_forwarding"></a><a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor Forwarding</a>
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<p>
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Consider writing a generic factory function that returns an object for a newly
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constructed generic type. Factory functions such as this are valuable for encapsulating
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and localizing the allocation of resources. Obviously, the factory function
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must accept exactly the same sets of arguments as the constructors of the type
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of objects constructed:
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</p>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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</pre>
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<p>
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Unfortunately, in C++03 the much bigger issue with this approach is that the
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N-argument case would require 2^N overloads, immediately discounting this as
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a general solution. Fortunately, most constructors take arguments by value,
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by const-reference or by rvalue reference. If these limitations are accepted,
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the forwarding emulation of a N-argument case requires just N overloads. This
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library makes this emulation easy with the help of <code class="computeroutput"><span class="identifier">BOOST_FWD_REF</span></code>
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and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span></code>:
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</p>
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<p>
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</p>
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<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>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">copyable_only_tester</span>
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<span class="special">{</span>
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<span class="keyword">public</span><span class="special">:</span>
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<span class="identifier">copyable_only_tester</span><span class="special">()</span>
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<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>
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<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>
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<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>
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<span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span>
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<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>
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<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>
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<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>
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<span class="special">};</span>
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<span class="keyword">class</span> <span class="identifier">copyable_movable_tester</span>
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<span class="special">{</span>
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<span class="comment">// move semantics</span>
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<span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">)</span>
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<span class="keyword">public</span><span class="special">:</span>
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<span class="identifier">copyable_movable_tester</span><span class="special">()</span>
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<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>
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<span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span>
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<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>
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<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>
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<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span>
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<span class="special">};</span>
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<span class="comment">//1 argument</span>
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<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>
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<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>
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<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>
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<span class="comment">//2 argument</span>
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<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>
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<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>
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<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>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">copyable_movable_tester</span> <span class="identifier">m</span><span class="special">;</span>
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<span class="comment">//move constructor</span>
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<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>
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<span class="comment">//copy constructor</span>
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<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>
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<span class="comment">//two rvalue constructor</span>
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<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>
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<span class="identifier">copyable_only_tester</span> <span class="identifier">nm</span><span class="special">;</span>
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<span class="comment">//copy constructor (copyable_only_tester has no move ctor.)</span>
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<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>
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<span class="comment">//copy constructor</span>
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<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>
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<span class="comment">//int constructor</span>
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<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>
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<span class="comment">//int, double constructor</span>
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<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>
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<span class="comment">//Output is:</span>
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<span class="comment">//copyable_movable_tester()</span>
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<span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))</span>
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<span class="comment">//copyable_movable_tester()</span>
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<span class="comment">//copyable_movable_tester(const copyable_movable_tester &)</span>
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<span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))</span>
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<span class="comment">//copyable_only_tester()</span>
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<span class="comment">//copyable_only_tester(const copyable_only_tester&)</span>
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<span class="comment">//copyable_only_tester(const copyable_only_tester&)</span>
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<span class="comment">//copyable_only_tester(int)</span>
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<span class="comment">//copyable_only_tester(int, double)</span>
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<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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||
<span class="special">}</span>
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||
</pre>
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<p>
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</p>
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<p>
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Constructor forwarding comes in handy to implement placement insertion in containers
|
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with just N overloads if the implementor accepts the limitations of this type
|
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of forwarding for C++03 compilers. In compilers with rvalue references perfect
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||
forwarding is achieved.
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||
</p>
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||
</div>
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||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<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>)
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</p>
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</div></td>
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</tr></table>
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