C++11新特性：enable_shared_from_this解决大问题

int main()
{
  int* pt = new int();
  shared_ptr<int> _p1(pt);
  shared_ptr<int> _p2(pt);
  std::cout << "_p1.use_count() = " << _p1.use_count() << std::endl;
  std::cout << " _p2.use_count() = " << _p2.use_count() << std::endl;
  return 0;
}

class MyCar:public std::enable_shared_from_this<MyCar>
{
public:
  shared_ptr<MyCar> get_ptr() {
    return shared_from_this();
  }
  ~MyCar() {
    std::cout << "free ~Mycar()" << std::endl;
  }
private:

};

int main()
{
  MyCar* _myCar = new MyCar();
  shared_ptr<MyCar> _myCar1(_myCar);
  shared_ptr<MyCar> _myCar2 = _myCar->get_ptr();
  std::cout << _myCar1.use_count() << std::endl;
  std::cout << _myCar2.use_count() << std::endl;
  return 0;
}

nt main()
{
  MyCar* _myCar = new MyCar();
  shared_ptr<MyCar> _myCar1 = _myCar->get_ptr();
  shared_ptr<MyCar> _myCar2 = _myCar->get_ptr();
  std::cout << _myCar1.use_count() << std::endl;
  std::cout << _myCar2.use_count() << std::endl;
  return 0;
}

// CLASS TEMPLATE enable_shared_from_this
template <class _Ty>
class enable_shared_from_this { // provide member functions that create shared_ptr to this
public:
    using _Esft_type = enable_shared_from_this;

    _NODISCARD shared_ptr<_Ty> shared_from_this() { // return shared_ptr
        return shared_ptr<_Ty>(_Wptr);
    }

    _NODISCARD shared_ptr<const _Ty> shared_from_this() const { // return shared_ptr
        return shared_ptr<const _Ty>(_Wptr);
    }

    _NODISCARD weak_ptr<_Ty> weak_from_this() noexcept { // return weak_ptr
        return _Wptr;
    }

    _NODISCARD weak_ptr<const _Ty> weak_from_this() const noexcept { // return weak_ptr
        return _Wptr;
    }

protected:
    constexpr enable_shared_from_this() noexcept : _Wptr() {}

    enable_shared_from_this(const enable_shared_from_this&) noexcept : _Wptr() {
        // construct (must value-initialize _Wptr)
    }

    enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept { // assign (must not change _Wptr)
        return *this;
    }

    ~enable_shared_from_this() = default;

private:
#if _HAS_IF_CONSTEXPR
    template <class _Yty>
    friend class shared_ptr;
#else // ^^^ _HAS_IF_CONSTEXPR // !_HAS_IF_CONSTEXPR vvv
    template <class _Other, class _Yty>
    friend void _Enable_shared_from_this1(const shared_ptr<_Other>& _This, _Yty* _Ptr, true_type) noexcept;
#endif // _HAS_IF_CONSTEXPR

    mutable weak_ptr<_Ty> _Wptr;
};