实现要点

  • shared_ptr 的引用计数应当使用原子操作（std::atomic）确保最低限度的并发安全。
  • 多个指向同一个底层指针对象的shared_ptr，它们的引用计数应当相等，因此应该将引用计数也定义成指针。
  • 使用移动赋值运算时，被移动的shared_ptr的引用计数应当减1，同时要判断是否释放资源（引用计数为0时）。
  • 使用移动构造函数时，要将被移动的对象的底层指针和引用计数器置空。
  • 重载解引用 * 和箭头 -> 运算符，实现类似普通指针的调用
  • 使用RAII机制自动释放资源

代码：

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template <typename T>
class SharedPtr
{
private:
    T *ptr;
    std::atomic<int> *count;

public:
    explicit SharedPtr(T *p = nullptr) : ptr(p), count(new std::atomic<int>(1))
    {
        if (p == nullptr)
        {
            count->store(0);
        }
    }

    SharedPtr(const SharedPtr<T> &other) : ptr(other.ptr), count(other.count)
    {
        count->fetch_add(1);
    }

    SharedPtr(SharedPtr<T> &&other) noexcept : ptr(other.ptr), count(other.count)
    {
        other.ptr = nullptr;
        other.count = nullptr;
    }

    SharedPtr<T> &operator=(const SharedPtr<T> &other)
    {
        if (this->ptr != other.ptr)
        {
            if (count->fetch_sub(1) == 0)
            {
                delete ptr;
                delete count;
            }
            this->ptr = other.ptr;
            this->count = other.count;
            if (this->ptr)
            {
                count->fetch_add(1);
            }
        }
    }

    SharedPtr<T> &operator=(SharedPtr<T> &&other)
    {
        if (this->ptr != other.ptr)
        {
            if (count->fetch_sub(1) == 0)
            {
                delete ptr;
                delete count;
            }
            this->ptr = other.ptr;
            this->count = other.count;
            other.ptr = nullptr;
            other.count = nullptr;
        }
    }

    ~SharedPtr()
    {
        if (count->fetch_sub(1) == 0)
        {
            delete ptr;
            delete count;
        }
    }

    T *operator->() const
    {
        return ptr;
    }

    T &operator*() const
    {
        return *ptr;
    }

    T *get() const noexcept
    {
        return ptr;
    }

    int use_count() const noexcept
    {
        return count->load();
    }

    bool unique() const noexcept
    {
        return count->load() == 1;
    }

    void swap(SharedPtr<T> &other) noexcept
    {
        std::swap(ptr, other.ptr);
        std::swap(count, other.count);
    }

    void reset(T *p = nullptr) noexcept
    {
        if (this->ptr != p)
        {
            if ((*count).fetch_sub(1) == 0)
            {
                delete ptr;
                delete count;
            }
            ptr = p;
            if (p != nullptr)
            {
                count = new std::atomic<int>(1);
            }
        }
    }
};