C++内存管理基础中shared_ptr的循环引用问题解决

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#include <iostream>
#include <memory>
#include <string>

// 前向声明，因为A和B互相引用
class B;

class A {
public:
    std::string name;
    std::shared_ptr<B> b_ptr; // A强引用B

    A(std::string n) : name(n) {
        std::cout << "A " << name << " constructor\n";
    }
    ~A() {
        std::cout << "A " << name << " destructor\n";
    }

    void set_b(std::shared_ptr<B> b) {
        b_ptr = b;
    }
};

class B {
public:
    std::string name;
    std::weak_ptr<A> a_ptr; // B弱引用A，这是打破循环的关键

    B(std::string n) : name(n) {
        std::cout << "B " << name << " constructor\n";
    }
    ~B() {
        std::cout << "B " << name << " destructor\n";
    }

    void set_a(std::shared_ptr<A> a) {
        a_ptr = a;
    }

    void check_a_status() {
        if (auto shared_a = a_ptr.lock()) { // 尝试提升为shared_ptr
            std::cout << "B " << name << " observes A " << shared_a->name << " is still alive.\n";
        } else {
            std::cout << "B " << name << " observes A is gone.\n";
        }
    }
};

int main() {
    std::cout << "--- 示例开始 ---\n";
    {
        std::shared_ptr<A> pa = std::make_shared<A>("Object_A");
        std::shared_ptr<B> pb = std::make_shared<B>("Object_B");

        std::cout << "初始引用计数：pa=" << pa.use_count() << ", pb=" << pb.use_count() << "\n";

        // 建立引用关系
        pa->set_b(pb); // A强引用B，B的引用计数增加
        pb->set_a(pa); // B弱引用A，A的引用计数不变

        std::cout << "建立引用后：pa=" << pa.use_count() << ", pb=" << pb.use_count() << "\n";
        // 此时，pa的use_count应为1 (main函数持有)，pb的use_count应为2 (main函数持有，A对象内部持有)

        pb->check_a_status(); // B尝试访问A，此时A应该还活着
    } // pa和pb超出作用域，智能指针自动析构
    std::cout << "--- 示例结束 ---\n";
    // 如果没有循环引用，A和B的析构函数都会被调用
    return 0;
}

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