🔴 循环引用：谁也不还

（本条路线统一用 g++ -std=c++17 -O0 -Wall 编译）上半段互相拿着，下半段把一边改成 weak_ptr：

#include <iostream>
#include <memory>

struct Res {
    static int gone;
    std::shared_ptr<Res> partner;     /* 互相拿着对方 —— 这就是循环 */
    ~Res() { gone++; }
};
int Res::gone = 0;

struct Res2 {
    static int gone;
    std::weak_ptr<Res2> partner;      /* 改成 weak，就不算一份 */
    ~Res2() { gone++; }
};
int Res2::gone = 0;

int main() {
    {
        auto a = std::make_shared<Res>();
        auto b = std::make_shared<Res>();
        a->partner = b;
        b->partner = a;
    }
    int cyc = Res::gone;              /* 两个都还没析构 */

    {
        auto a = std::make_shared<Res2>();
        auto b = std::make_shared<Res2>();
        a->partner = b;
        b->partner = a;
    }
    int fixed = Res2::gone;

    std::cout << cyc << "/" << fixed << "\n";
    return 0;
}