拷贝和移动

from https://zh.cppreference.com/w/cpp/header/vector

class vector {
  private:
    // ......
  public:
    vector() = default;
    ~vector() = default;
    // copy
    vector(const vector& x) = default;
    vector& operator=(const vector& x) = default;
    // move
    vector(vector&&) = default;
    vector& operator=(vector&& x) = default;
  public:
    // ......
};