shared_ptr智能指针为什么循环引用会出问题

 1 #include <iostream>
 2 #include <typeinfo>
 3 
 4 using namespace std;
 5 
 6 template <typename T>
 7 class SharedPointer {
 8 private:
 9     class Implement    {
10     public:
11         Implement(T* p) : mPointer(p), mRefs(1) {
12             cout << "Implement()" << endl;
13         }
14 
15         ~Implement(){
16             delete mPointer;
17             cout << "~Implement()" << endl;
18         }
19 
20         T* mPointer;
21         size_t mRefs;
22     };
23 
24     Implement* mImplPtr;
25 
26 public:
27     explicit SharedPointer(T* p = nullptr)
28         : mImplPtr(new Implement(p)) {
29         cout << "SharedPointer<" << typeid(T).name() << ">(" << this << ")" << endl;
30     }
31 
32     ~SharedPointer() {
33         cout << "~SharedPointer<" << typeid(T).name() << ">(" << this << ")" << endl;
34         decrease();
35     }
36 
37     SharedPointer(const SharedPointer& other)
38         : mImplPtr(other.mImplPtr) {
39         increase();
40         cout << "SharedPointer<" << typeid(T).name() << ">(other=" << &other << ")" << endl;
41     }
42 
43     SharedPointer& operator = (const SharedPointer& other) {
44         if(mImplPtr != other.mImplPtr) {
45             decrease();
46             mImplPtr = other.mImplPtr;
47             increase();
48         }
49 
50         return *this;
51     }
52 
53     T* operator -> () const    {
54         return mImplPtr->mPointer;
55     }
56 
57     T& operator * () const {
58         return *(mImplPtr->mPointer);
59     }
60 
61 private:
62     void decrease()    {
63         if(--(mImplPtr->mRefs) == 0) {
64             delete mImplPtr;
65         }
66     }
67 
68     void increase()    {
69         ++(mImplPtr->mRefs);
70     }
71 };
72 
73 class B;
74 
75 class A {
76 public:
77     SharedPointer<B> m_ptr;
78 };
79 
80 class B {
81 public:
82     SharedPointer<A> m_ptr;
83 };
84 
85 int main() {
86     SharedPointer<A> a(new A);
87     SharedPointer<B> b(new B);
88     a->m_ptr = b;
89     b->m_ptr = a;
90 
91     return 0;
92 }

 1 Implement()
 2 SharedPointer<1B>(0x417eb0)
 3 Implement()
 4 SharedPointer<1A>(0x7fff4fd10230)
 5 Implement()
 6 SharedPointer<1A>(0x418f20)
 7 Implement()
 8 SharedPointer<1B>(0x7fff4fd10218)
 9 ~Implement()
10 ~Implement()
11 ~SharedPointer<1B>(0x7fff4fd10218)
12 ~SharedPointer<1A>(0x7fff4fd10230)