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POMultiple copy constructors specified
 

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    <p>With Visual C++ 2010, I have a class like this:</p> <pre><code>class MyClass{ public: MyClass(){} MyClass(MyClass &amp;){/*...*/} //A MyClass(const MyClass &amp;){/*...*/} //B template&lt;typename T&gt; MyClass(T &amp;&amp;t){ static_assert( !std::is_same&lt;typename std::remove_cv&lt;typename std::remove_reference&lt;T&gt;::type&gt;::type, MyClass&gt;::value, "Wrapping over wrapping is not allowed!"); } //C }; int main(int, char**){ MyClass a; const MyClass b(a); //assert fail if line A removed auto c=b; //assert fail if line B removed } //If both A and B exists //Warning C4521: multiple copy constructors specified //Furthermore, if both A and B removed //No error or warnings; VC2010 accepts peacefully. //In debug mode you will find the compiler generated trivial copy constructor </code></pre> <p>According to C++ standard, both line A and B are considered copy constructors, and C is a convert constructor. It is not surprising that I receive a warning that I declared multiple copy constructors. However, if I remove any of them, the static_assert fails and the code would not compile, that means the template constructor received control.</p> <p>I am sure that this behaviour follows the rule of function overloading. However is this a conflict of two rules? If A and B are copy constructors and one of them was declared, any attempt to copy the objects should not drop to the template, is it right?</p> <p>Update: According to N3242, 12.8.7,</p> <blockquote> <p>"a member function template is NEVER INSTANTIATED to perform the copy of a class object to an object of its class type."</p> </blockquote> <p>the correct implementation should be:</p> <ul> <li>No assert failure should occur when either A or B or both removed.</li> <li>If line B is removed, construction of c should fail because b is const.</li> <li>If both lines are removed, compiler should generate a copy constructor for the class.</li> <li>It is up to the implementation to warn the user if both lines exists.</li> </ul> <p>Any comment?</p>
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    1. COYou've brought this on yourself with the inclusion of the `static_assert`. I can think of no good reason to not allow conversion from type `T&` to type `const T&` - can you?
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    2. COThis is just a dummy implementation of my class. The template constructor should have different logic than the copy constructor.
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    3. COTo implement the template convert constructor, usually a contained storage should be used to store the value of t. If t is instance of MyClass, it would introduce wrapping over wrapping, which is what I am tring to prevent.
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