c++新特性11

template <class _Ty>
class _Ptr_base { // base class for shared_ptr and weak_ptr
public:
    using element_type = remove_extent_t<_Ty>;

    _NODISCARD long use_count() const noexcept {
        return _Rep ? _Rep->_Use_count() : 0;
    }

    template <class _Ty2>
    _NODISCARD bool owner_before(const _Ptr_base<_Ty2>& _Right) const noexcept { // compare addresses of manager objects
        return _Rep < _Right._Rep;
    }

    _Ptr_base(const _Ptr_base&) = delete;
    _Ptr_base& operator=(const _Ptr_base&) = delete;

protected:
    _NODISCARD element_type* get() const noexcept {
        return _Ptr;
    }
 
    template <class _Ty2>
    void _Move_construct_from(_Ptr_base<_Ty2>&& _Right) noexcept {
        // implement shared_ptr's (converting) move ctor and weak_ptr's move ctor
        _Ptr = _Right._Ptr;
        _Rep = _Right._Rep;

        _Right._Ptr = nullptr;
        _Right._Rep = nullptr;
    }

    template <class _Ty2>
    void _Copy_construct_from(const shared_ptr<_Ty2>& _Other) noexcept {
        // implement shared_ptr's (converting) copy ctor
        _Other._Incref();

        _Ptr = _Other._Ptr;
        _Rep = _Other._Rep;
    }

    template <class _Ty2>
    void _Alias_construct_from(const shared_ptr<_Ty2>& _Other, element_type* _Px) noexcept {
        // implement shared_ptr's aliasing ctor
        _Other._Incref();

        _Ptr = _Px;
        _Rep = _Other._Rep;
    }

    template <class _Ty2>
    void _Alias_move_construct_from(shared_ptr<_Ty2>&& _Other, element_type* _Px) noexcept {
        // implement shared_ptr's aliasing move ctor
        _Ptr = _Px;
        _Rep = _Other._Rep;

        _Other._Ptr = nullptr;
        _Other._Rep = nullptr;
    }

    template <class _Ty0>
    friend class weak_ptr; // specifically, weak_ptr::lock()

    template <class _Ty2>
    bool _Construct_from_weak(const weak_ptr<_Ty2>& _Other) noexcept {
        // implement shared_ptr's ctor from weak_ptr, and weak_ptr::lock()
        if (_Other._Rep && _Other._Rep->_Incref_nz()) {
            _Ptr = _Other._Ptr;
            _Rep = _Other._Rep;
            return true;
        }

        return false;
    }

    void _Incref() const noexcept {
        if (_Rep) {
            _Rep->_Incref();
        }
    }

    void _Decref() noexcept { // decrement reference count
        if (_Rep) {
            _Rep->_Decref();
        }
    }

    void _Swap(_Ptr_base& _Right) noexcept { // swap pointers
        _STD swap(_Ptr, _Right._Ptr);
        _STD swap(_Rep, _Right._Rep);
    }

    template <class _Ty2>
    void _Weakly_construct_from(const _Ptr_base<_Ty2>& _Other) noexcept { // implement weak_ptr's ctors
        if (_Other._Rep) {
            _Ptr = _Other._Ptr;
            _Rep = _Other._Rep;
            _Rep->_Incwref();
        } else {
            _STL_INTERNAL_CHECK(!_Ptr && !_Rep);
        }
    }

    template <class _Ty2>
    void _Weakly_convert_lvalue_avoiding_expired_conversions(const _Ptr_base<_Ty2>& _Other) noexcept {
        // implement weak_ptr's copy converting ctor
        if (_Other._Rep) {
            _Rep = _Other._Rep; // always share ownership
            _Rep->_Incwref();

            if (_Rep->_Incref_nz()) {
                _Ptr = _Other._Ptr; // keep resource alive during conversion, handling virtual inheritance
                _Rep->_Decref();
            } else {
                _STL_INTERNAL_CHECK(!_Ptr);
            }
        } else {
            _STL_INTERNAL_CHECK(!_Ptr && !_Rep);
        }
    }

    template <class _Ty2>
    void _Weakly_convert_rvalue_avoiding_expired_conversions(_Ptr_base<_Ty2>&& _Other) noexcept {
        // implement weak_ptr's move converting ctor
        _Rep        = _Other._Rep; // always transfer ownership
        _Other._Rep = nullptr;

        if (_Rep && _Rep->_Incref_nz()) {
            _Ptr = _Other._Ptr; // keep resource alive during conversion, handling virtual inheritance
            _Rep->_Decref();
        } else {
            _STL_INTERNAL_CHECK(!_Ptr);
        }

        _Other._Ptr = nullptr;
    }

    void _Incwref() const noexcept {
        if (_Rep) {
            _Rep->_Incwref();
        }
    }

    void _Decwref() noexcept { // decrement weak reference count
        if (_Rep) {
            _Rep->_Decwref();
        }
    }

private:
    element_type* _Ptr{nullptr};
    _Ref_count_base* _Rep{nullptr};    
};