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POimplicit vs. explicit deleted copy constructor
 

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    <p>I've recently experienced some strange behaviour in my C++11 code.</p> <p>I've got a class, which is only movable:</p> <pre><code>class only_move { public: only_move() : value(0) {} only_move(int value) : value(value) {} only_move(const only_move&amp;) = delete; only_move&amp; operator=(const only_move&amp;) = delete; only_move(only_move&amp;&amp;) = default; only_move&amp; operator=(only_move&amp;&amp;) = default; int value; }; </code></pre> <p>And I've got another class, which contains an <code>only_move</code> object:</p> <pre><code>class has_only_move_member { public: has_only_move_member() = delete; has_only_move_member(only_move&amp;&amp; value) : member(std::move(value)) {} only_move member; }; </code></pre> <p>If I understood it correct, that means <code>has_only_move_member</code> can't be copied, as the <code>only_move</code> member can't be copied. This means <code>has_only_move_member(const has_only_move_member&amp;)</code> is deleted implicitly. Let's check that out:</p> <pre><code>has_only_move_member object(only_move(5)); has_only_move_member copy_of_object(object); </code></pre> <p>As expected, it prints out:</p> <pre><code>error: use of deleted function ‘has_only_move_member::has_only_move_member(const has_only_move_member&amp;)’ note: ‘has_only_move_member::has_only_move_member(const has_only_move_member&amp;)’ is implicitly deleted because the default definition would be ill-formed: class has_only_move_member </code></pre> <p>Okay, so I moved on and put <code>has_only_move_member</code> instances in a <code>std::map</code>. As they don't have a copy constructor, I've moved them into it:</p> <pre><code>has_only_move_member object(only_move(5)); std::map&lt;int, has_only_move_member&gt; data; data.emplace(5, std::move(object)); </code></pre> <p>So far so good. This works like a charm.</p> <p>But I had an idea. What about being more explicit and explicitly deleting the copy constructor of <code>has_only_move_member</code>. So I wrote into the class <code>has_only_move_member</code>:</p> <pre><code>has_only_move_member(const has_only_move_member&amp;) = delete; </code></pre> <p>After that, the same code from above, there I moved objects into an map gives me an error:</p> <pre><code>/usr/include/c++/4.8/bits/stl_pair.h:134:45: error: use of deleted function ‘has_only_move_member::has_only_move_member(const has_only_move_member&amp;)’ </code></pre> <p>Yeah of course it's deleted, but why is there such a gap between implicit and explicit delete?</p> <p>I'm getting the same behaviour with g++ 4.8.2 and clang 3.4-rc1 on Debian with libstdc++ and recent apple-clang-llvm 4.0 on Mac OS X 10.9 with libc++</p>
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    1. COYou added a custom constructor (even though it's a deleted one). Therefore it won't auto generate a move constructor. It then falls back to copying which it obviously can't do.
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    2. COOh, shame on me. How could I forget this? ;-( Thank you.
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