struct SharedPtr
{
	template<class P>
	struct Wrapper
	{
		P* ptr;
		long count;

		Wrapper(P* ptr) : ptr(ptr), count(1)
		{ if (ptr == nullptr ) throw "Invalid pointer"; }

		~Wrapper() { delete ptr; }

		void addRef() { ++count; }
		void release()
		{
			if (--count == 0)
			{
				delete this;
			}
		}
	};

    explicit SharedPtr(Expression *ptr = nullptr)
    	: holder(ptr ? new Wrapper<Expression>(ptr) : nullptr)
    {
    }

    SharedPtr(const SharedPtr& other) : holder(nullptr)
    {
    	copy(other);
    }

    ~SharedPtr()
    {
    	clear();
    }

    SharedPtr& operator=(const SharedPtr& other)
    {
        if (this != &other)
        {
        	copy(other);
        }
        return *this;
    }

    Expression* get() const { return holder ? holder->ptr : nullptr;}

    void reset(Expression *ptr = nullptr)
    {
    	clear();

    	if (ptr != nullptr)
    	{
    		holder = new Wrapper<Expression>(ptr);
    	}
    }

    const Expression& operator*() const
    {
    	if (holder == nullptr) throw "Invalid pointer";
    	return *holder->ptr;
    }
    Expression& operator*()
    {
    	if (holder == nullptr) throw "Invalid pointer";
    	return *holder->ptr;
    }

    Expression* operator->() const
    {
    	return get();
    }

private:
    void copy(const SharedPtr& other)
    {
    	clear();

    	if (other.holder)
		{
			holder = other.holder;
			holder->addRef();
		}
    }

    void clear()
    {
    	if (holder)
		{
			holder->release();
			holder = nullptr;
		}
    }

    Wrapper<Expression> *holder;
};