class A{
    std::vector<int> vec;
public:
    A() = default;
    A(const A &) = default;
    A(A && o) : vec(std::move(o.vec)) {}

    const A & operator = (A && o) {
        vec = std::move(o.vec);
        return *this;
    }
    const A & operator = (A o) {
        std::swap(*this, o);
        return *this;
    }
};