Smart pointer examples 
void testRefCount(){
    shared_ptr<Node> pt0(new Node(1));
    shared_ptr<Node> pt1 = pt0;
    shared_ptr<Node> pt2 = pt0;
    printf("count=[%ld]\n", pt0.use_count()); // 3
    pt1.reset();
    printf("count=[%ld]\n", pt0.use_count()); // 2
    pt2.reset();
    printf("count=[%ld]\n", pt0.use_count()); // 1
    pt0.reset();
    printf("count=[%ld]\n", pt0.use_count()); // 0 
}

Cycle in shared_ptr 
class Class2;
class Class1{
    public:
    shared_ptr<Class2> c2;
    //weak_ptr<Class2> c2;
    public:
    ~Class1(){
        printf("Destroy Class1()\n");
    }
};

class Class2{
    public:
        shared_ptr<Class1> c1;
    public:
    ~Class2(){
        printf("Destroy Class2()\n");
    }
};

void cycle(){
    auto c1 = make_shared<Class1>();
    auto c2 = make_shared<Class2>();
    c1->c2 = c2;
    c2->c1 = c1;
}
Break cycle with weak_ptr 
class Class2;
class Class1{
    public:
    //shared_ptr<Class2> c2;
    weak_ptr<Class2> c2;
    public:
    ~Class1(){
        printf("Destroy Class1()\n");
    }
};

class Class2{
    public:
        shared_ptr<Class1> c1;
    public:
    ~Class2(){
        printf("Destroy Class2()\n");
    }
};

void cycle(){
    auto c1 = make_shared<Class1>();
    auto c2 = make_shared<Class2>();
    c1->c2 = c2;
    c2->c1 = c1;
}

Use smart pointer in return 
Node* bad_idea(){
    return new Node(1);
}

shared_ptr<Node> good_idea(){
    shared_ptr<Node> pt(new Node(1));
    return pt;
}
unique_ptr transfers ownership 
void uniqueTransferOwnerShip(){
    unique_ptr<Node> upt(new Node(1)); 
    // error
    // unique_ptr<Node> upt1 = upt;
    // error
    // unique_ptr<Node> upt2(upt);
    unique_ptr<Node> upt3 = std::move(upt);
}

How to use weak pointer
1. Use weak pointer to check whether a pointer still exists or not 
                int* pt1 = new int(10);
                int* pt2 = pt1;
                delete pt1;
                
                // pt2 is danging pointer

                // how to use weak pointer to solve it
                shared_ptr spt1;
                spt1.reset(new int(100));

                weak_ptr wptr = spt1;

                if(auto tmp = wptr.lock())
                    cout<<"wptr is not expired yet"<
        

        

        
 
        

        Source Code