Does Visual Studio 2017 need an explicit move constructor declaration?

Question

The below code can be compiled successfully using Visual Studio 2015, but it failed using Visual Studio 2017. Visual Studio 2017 reports:

error C2280:

Answer 1

Let's look at the std::vector source code (I replaced pointer and _Ty with actual types):

void _Umove_if_noexcept1(Node* First, Node* Last, Node* Dest, true_type)
    {   // move [First, Last) to raw Dest, using allocator
    _Uninitialized_move(First, Last, Dest, this->_Getal());
    }

void _Umove_if_noexcept1(Node* First, Node* Last, Node* Dest, false_type)
{   // copy [First, Last) to raw Dest, using allocator
    _Uninitialized_copy(First, Last, Dest, this->_Getal());
}

void _Umove_if_noexcept(Node* First, Node* Last, Node* Dest)
{   // move_if_noexcept [First, Last) to raw Dest, using allocator
    _Umove_if_noexcept1(First, Last, Dest,
        bool_constant, negation>>>{});
}

If Node is no-throw move-constructible or is not copy-constructible, _Uninitialized_move is called, otherwise, _Uninitialized_copy is called.

The problem is that the type trait std::is_copy_constructible_v is true for Node if you do not declare a move constructor explicitly. This declaration makes copy-constructor deleted.

libstdc++ implements std::vector in a similar way, but there std::is_nothrow_move_constructible_v is true in contrast to MSVC, where it is false. So, move semantics is used and the compiler does not try to generate the copy-constructor.

But if we force is_nothrow_move_constructible_v to become false

struct Base {
    Base() = default;
    Base(const Base&) = default;
    Base(Base&&) noexcept(false) { }
};

struct Node : Base {
    std::unordered_map> map;
};

int main() {
    std::vector vec;
    vec.reserve(1);
}

the same error occurs:

/usr/include/c++/7/ext/new_allocator.h:136:4: error: use of deleted function ‘std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = std::unique_ptr]’
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~