class CB;
class CA
{
public:
    CA() { cout << "CA() called! " << endl; }
    ~CA() { cout << "~CA() called! " << endl; }
    void set_ptr(shared_ptr<CB>& ptr) { m_ptr_b = ptr; }
    void b_use_count() { cout << "b use count : " << m_ptr_b.use_count() << endl; }
    void show() { cout << "this is class CA!" << endl; }
private:
    shared_ptr<CB> m_ptr_b;
};
class CB
{
public:
    CB() { cout << "CB() called! " << endl; }
    ~CB() { cout << "~CB() called! " << endl; }
    void set_ptr(shared_ptr<CA>& ptr) { m_ptr_a = ptr; }
    void a_use_count() { cout << "a use count : " << m_ptr_a.use_count() << endl; }
    void show() { cout << "this is class CB!" << endl; }
private:
    shared_ptr<CA> m_ptr_a;
};
void test_refer_to_each_other()
{
    shared_ptr<CA> ptr_a(new CA());
    shared_ptr<CB> ptr_b(new CB());
    cout << "a use count : " << ptr_a.use_count() << endl;
    cout << "b use count : " << ptr_b.use_count() << endl;
    ptr_a->set_ptr(ptr_b);
    ptr_b->set_ptr(ptr_a);
    cout << "a use count : " << ptr_a.use_count() << endl;
    cout << "b use count : " << ptr_b.use_count() << endl;
}

class CB
{
public:
    CB() { cout << "CB() called! " << endl; }
    ~CB() { cout << "~CB() called! " << endl; }
    void set_ptr(shared_ptr<CA>& ptr) { m_ptr_a = ptr; }
    void a_use_count() { cout << "a use count : " << m_ptr_a.use_count() << endl; }
    void show() { cout << "this is class CB!" << endl; }
private:
    weak_ptr<CA> m_ptr_a;
};

CA() called!
CB() called!
a use count : 1
b use count : 1
a use count : 1
b use count : 2
~CA() called!
~CB() called!

void test1()
{
    // 編譯錯(cuò)誤 // error C2665: “std::weak_ptr<CA>::weak_ptr”: 3 個(gè)重載中沒(méi)有一個(gè)可以轉(zhuǎn)換所有參數(shù)類型
    // weak_ptr<CA> ptr_1(new CA());
    shared_ptr<CA> ptr_1(new CA());
    cout << "ptr_1 use count : " << ptr_1.use_count() << endl; // 輸出：ptr_1 use count : 1
    shared_ptr<CA> ptr_2 = ptr_1;
    cout << "ptr_1 use count : " << ptr_1.use_count() << endl; // 輸出：ptr_1 use count : 2
    cout << "ptr_2 use count : " << ptr_2.use_count() << endl; // 輸出：ptr_1 use count : 2
    weak_ptr<CA> wk_ptr = ptr_1;
    cout << "ptr_1 use count : " << ptr_1.use_count() << endl; // 輸出：ptr_1 use count : 2
    cout << "ptr_2 use count : " << ptr_2.use_count() << endl; // 輸出：ptr_1 use count : 2
    // 編譯錯(cuò)誤
    // error C2440 : “初始化”: 無(wú)法從“std::weak_ptr<CA>”轉(zhuǎn)換為“std::shared_ptr<CA>”
    // shared_ptr<CA> ptr_3 = wk_ptr;
}

void test2()
{
    shared_ptr<CA> ptr_a(new CA());     // 輸出：CA() called!
    shared_ptr<CB> ptr_b(new CB());     // 輸出：CB() called!
    cout << "ptr_a use count : " << ptr_a.use_count() << endl; // 輸出：ptr_a use count : 1
    cout << "ptr_b use count : " << ptr_b.use_count() << endl; // 輸出：ptr_b use count : 1
    weak_ptr<CA> wk_ptr_a = ptr_a;
    weak_ptr<CB> wk_ptr_b = ptr_b;
    if (!wk_ptr_a.expired())
    {
        wk_ptr_a.lock()->show();        // 輸出：this is class CA!
    }
    if (!wk_ptr_b.expired())
    {
        wk_ptr_b.lock()->show();        // 輸出：this is class CB!
    }
    // 編譯錯(cuò)誤
    // 編譯必須作用于相同的指針類型之間
    // wk_ptr_a.swap(wk_ptr_b);         // 調(diào)用交換函數(shù)
    wk_ptr_b.reset();                   // 將wk_ptr_b的指向清空
    if (wk_ptr_b.expired())
    {
        cout << "wk_ptr_b is invalid" << endl;  // 輸出：wk_ptr_b is invalid 說(shuō)明改指針已經(jīng)無(wú)效
    }
    wk_ptr_b = ptr_b;
    if (!wk_ptr_b.expired())
    {
        wk_ptr_b.lock()->show();        // 輸出：this is class CB! 調(diào)用賦值操作后，wk_ptr_b恢復(fù)有效
    }
    // 編譯錯(cuò)誤
    // 編譯必須作用于相同的指針類型之間
    // wk_ptr_b = wk_ptr_a;

    // 最后輸出的引用計(jì)數(shù)還是1，說(shuō)明之前使用weak_ptr類型賦值，不會(huì)影響引用計(jì)數(shù)
    cout << "ptr_a use count : " << ptr_a.use_count() << endl; // 輸出：ptr_a use count : 1
    cout << "ptr_b use count : " << ptr_b.use_count() << endl; // 輸出：ptr_b use count : 1
}