智能指针引用计数

	class Demo : public std::enable_shared_from_this<Demo> {
    public:
        std::string key;
        std::string value;
    };
    std::shared_ptr<Demo> A = std::shared_ptr<Demo>(new Demo());
    int count1 = A.use_count();//1
    std::shared_ptr<Demo> B = A;
    int count2 = A.use_count();//2
    int count3 = B.use_count();//2
    std::weak_ptr<Demo> C = A;
    int count4 = A.use_count();//2
    int count5 = B.use_count();//2
    int count6 = C.use_count();//2
    std::shared_ptr<Demo> D = C.lock();
    int count7 = A.use_count();//3
    int count8 = B.use_count();//3
    int count9 = C.use_count();//3
    int count10 = D.use_count();//3
    std::shared_ptr<Demo> E = std::shared_ptr<Demo>(A.get()); //这种写法会导致内存空间重复释放
    int count11 = A.use_count();//3
    int count12 = B.use_count();//3
    int count13 = C.use_count();//3
    int count14 = D.use_count();//3
    int count15 = E.use_count();//1
    std::shared_ptr<Demo> F = A->shared_from_this();
    int count16 = A.use_count();//4
    int count17 = B.use_count();//4
    int count18 = C.use_count();//4
    int count19 = D.use_count();//4
    int count20 = E.use_count();//1
    int count21 = F.use_count();//4
    std::weak_ptr<Demo> G = A->shared_from_this();
    int count22 = A.use_count();//4
    int count23 = B.use_count();//4
    int count24 = C.use_count();//4
    int count25 = D.use_count();//4
    int count26 = E.use_count();//1
    int count27 = F.use_count();//4
    int count28 = G.use_count();//4

    A = nullptr;
    int count29 = A.use_count();//0
    int count30 = B.use_count();//3
    int count31 = C.use_count();//3
    int count32 = D.use_count();//3
    int count33 = E.use_count();//1
    int count34 = F.use_count();//3
    int count35 = G.use_count();//3