智能指针weak_ptr使用

weak_ptr 的作用就是为了解决 shared_ptr 循环引用的问题，原理是对于shared_ptr 的指针采用只引用不计数的方法。

weak_ptr 的初始化只能是 weak_ptr 和 shared_ptr 的类型数据。

weak_ptr 使用之前需要做安全检测，expired() 判断是否有效，返回类型为bool值

lock返回值是一个 shared_ptr，如果内存已经被释放了，则会返回一个null。

#include<iostream>
#include<memory>
using namespace std;

class A {
private:
    int a;
    int b;
public:
    A(){};
    A(int a, int b):a(a), b(b){};
    int get_a() {
        return a;
    };
    int get_b() {
        return b;
    }
    void set_a(int val) {
        a = val;
    }
    void set_b(int val) {
        b = val;
    }
};

int main() {
    shared_ptr<A> pa(new A());
    shared_ptr<A> pb = make_shared<class A>(1,2);
    pa->set_a(3);
    cout << pa->get_a() << endl;    //3
    cout << pa->get_b() << endl;    //0

    cout << pb->get_a() << endl;    //1
    cout << pb->get_b() << endl;    //2

    cout << pa.use_count() << endl; //1

    weak_ptr<A> wa = pa;
    cout << wa.use_count() << endl; //1
    cout << pa.use_count() << endl; //1

    cout << wa.lock()->get_a() << endl; //3
    cout << wa.use_count() << endl;     //1
    cout << pa.use_count() << endl;     //1

    auto p = wa.lock();
    cout << p->get_b() << endl;    // 0
    cout << wa.use_count() << endl;    //2
    cout << pa.use_count() << endl;     //2  

    weak_ptr<A> wb(pb);
    pb.reset();
    if (wb.lock() == nullptr) {
        cout << "pb is expired" << endl;
    }
    else {
        cout << wb.lock()->get_a() << endl;
    }

    pa.reset();
    if (wa.expired()) {
        cout << wa.lock()->get_a() << endl;
    }
    else {
        cout << "pa is expired" << endl;
    }

    return 0;
}