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POHow should I assign boost::interprocess::unique_ptr returned from a factory function, possibly using boost::move, in C++03
 

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  1. POHow should I assign boost::interprocess::unique_ptr returned from a factory function, possibly using boost::move, in C++03
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    <p>I'm trying to create a factory function that would return boost::interprocess::unique_ptr. Here's an example:</p> <pre><code>#include &lt;boost/interprocess/smart_ptr/unique_ptr.hpp&gt; using namespace boost::interprocess; class my_class { public: my_class() {} }; struct my_class_deleter { void operator()(my_class *p) {} }; typedef unique_ptr&lt;my_class, my_class_deleter&gt; uptr; uptr create() { return uptr(); } int main() { uptr x; x = create(); return 0; } </code></pre> <p>The problem is that gcc fails to compile the above code saying:</p> <pre><code>main.cpp:22: error: ambiguous overload for ‘operator=’ in ‘x = create()()’ ../../boost_latest/boost/interprocess/smart_ptr/unique_ptr.hpp:211: note: candidates are: boost::interprocess::unique_ptr&lt;T, D&gt;&amp; boost::interprocess::unique_ptr&lt;T, D&gt;::operator=(boost::rv&lt;boost::interprocess::unique_ptr&lt;T, D&gt; &gt;&amp;) [with T = my_class, D = my_class_deleter] ../../boost_latest/boost/interprocess/smart_ptr/unique_ptr.hpp:249: note: boost::interprocess::unique_ptr&lt;T, D&gt;&amp; boost::interprocess::unique_ptr&lt;T, D&gt;::operator=(int boost::interprocess::unique_ptr&lt;T, D&gt;::nat::*) [with T = my_class, D = my_class_deleter] </code></pre> <p>When I change</p> <pre><code>x = create(); </code></pre> <p>to</p> <pre><code>x = boost::move(create()); </code></pre> <p>then gcc says:</p> <pre><code>main.cpp:22: error: invalid initialization of non-const reference of type ‘uptr&amp;’ from a temporary of type ‘uptr’ ../../boost_latest/boost/move/move.hpp:330: error: in passing argument 1 of ‘typename boost::move_detail::enable_if&lt;boost::has_move_emulation_enabled&lt;T&gt;, boost::rv&lt;T&gt;&amp;&gt;::type boost::move(T&amp;) [with T = uptr]’ </code></pre> <p>Am I doing something wrong?</p> <p>Interestingly, when I do:</p> <pre><code>uptr x2 = create(); </code></pre> <p>the code compiles without any issues.</p> <p>BTW: I use gcc v4.4.3 and Boost v1.51.0.</p> <hr> <p><strong>UPDATE:</strong></p> <p>I've been able to overcome this issue by using the following snippet:</p> <pre><code>x = static_cast&lt;boost::rv&lt;uptr&gt;&amp;&gt;(create()); </code></pre> <p>The above cast is based on the first version of ambiguous overload for <code>operator=</code> mentioned in the original question. The second one (<code>operator=(int boost::interprocess::unique_ptr&lt;T, D&gt;::nat::*</code>) is probably provided by the implementation to emulate <code>std::unique_ptr::operator=(nullptr_t)</code>, which as a matter of fact resets the <code>unique_ptr</code>. It turns out, it also makes <code>operator=</code> ambiguous.</p> <p>Unfortunately, using the above-mentioned <code>static_cast&lt;&gt;()</code> makes using my factory too much complicated.</p> <p>One way to solve this problem would be to remove the second overload for <code>operator=</code>, as one can always explicitly call <code>unique_ptr::reset()</code>.</p> <p>Still, I'm wondering if and how <code>boost::move()</code> could help me with this issue.</p>
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