c++新特性11 (10)shared_ptr二”_Ref_count_base类“

Posted 2022-04-01 thefist11

. 保证线程安全
. 析构时如果计数器为0，删除自身

// CLASS _Ref_count_base
class __declspec(novtable) _Ref_count_base  // common code for reference counting
private:

    _Atomic_counter_t _Uses  = 1;
    _Atomic_counter_t _Weaks = 1;

protected:
    constexpr _Ref_count_base() noexcept = default; // non-atomic initializations

public:
    _Ref_count_base(const _Ref_count_base&) = delete;
    _Ref_count_base& operator=(const _Ref_count_base&) = delete;


    bool _Incref_nz() noexcept  // increment use count if not zero, return true if successful
        auto& _Volatile_uses = reinterpret_cast<volatile long&>(_Uses);
#ifdef _M_CEE_PURE
        long _Count = *_Atomic_address_as<const long>(&_Volatile_uses);
#else
        long _Count = __iso_volatile_load32(reinterpret_cast<volatile int*>(&_Volatile_uses));
#endif
        while (_Count != 0) 
            const long _Old_value = _INTRIN_RELAXED(_InterlockedCompareExchange)(&_Volatile_uses, _Count + 1, _Count);
            if (_Old_value == _Count) 
                return true;
            

            _Count = _Old_value;
        

        return false;
    

    void _Incref() noexcept  // increment use count
        _MT_INCR(_Uses);
    

    void _Incwref() noexcept  // increment weak reference count
        _MT_INCR(_Weaks);
    

    void _Decref() noexcept  // decrement use count
        if (_MT_DECR(_Uses) == 0) 
            _Destroy();
            _Decwref();
        
    

    void _Decwref() noexcept  // decrement weak reference count
        if (_MT_DECR(_Weaks) == 0) 
            _Delete_this();
        
    

    long _Use_count() const noexcept 
        return static_cast<long>(_Uses);
    

    virtual void* _Get_deleter(const type_info&) const noexcept 
        return nullptr;