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    <p>Implicitly generated move constructors have been considered for the standard, but can be dangerous. See Dave Abrahams's <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3153.htm" rel="noreferrer">analysis</a>.</p> <p>In the end, however, the standard did include implicit generation of move constructors and move assignment operators, though with a fairly substantial list of limitations:</p> <blockquote> <p>If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if<br> — X does not have a user-declared copy constructor,<br> — X does not have a user-declared copy assignment operator,<br> — X does not have a user-declared move assignment operator,<br> — X does not have a user-declared destructor, and<br> — the move constructor would not be implicitly defined as deleted.</p> </blockquote> <p>That's not quite all there is to the story though. A ctor can be declared, but still defined as deleted:</p> <blockquote> <p>An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted (8.4.3) if X has:</p> <p>— a variant member with a non-trivial corresponding constructor and X is a union-like class,<br> — a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,<br> — a direct or virtual base class B that cannot be copied/moved because overload resolution (13.3), as applied to B’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,<br> — any direct or virtual base class or non-static data member of a type with a destructor that is deleted or inaccessible from the defaulted constructor,<br> — for the copy constructor, a non-static data member of rvalue reference type, or<br> — for the move constructor, a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable.</p> </blockquote>
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    1. POWhy no default move-assignment/move-constructor?
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    1. USJerry Coffin
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    1. COThe current working draft does allow implicit move generation under certain conditions and I think the resolution largely addresses Abrahams's concerns.
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      1. USJames McNellis
    2. COI am not sure I understood what move may break in the example between Tweak 2 and Tweak 3. Could you explain it ?
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      1. USMatthieu M.
    3. CO@Matthieu M.: both Tweak 2 and Tweak 3 are broken, and in pretty similar ways, really. In Tweak 2, there are private members with invariants that can be broken by the move ctor. In Tweak 3, the class doesn't have private members *itself*, but since it uses private inheritance, the public and protected members of the base become private members of the derived, leading to the same problem.
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