共享指针循环引用使用弱指针解决

#include <iostream>
#include <memory>
#include <string>
/// @brief 手写共享指针
/// @tparam T 指针内存类型
template <class T>
class shared_ptr_t {
private:
    T*   ptr_;
    int* count_;

private:
    /// @brief 释放内存
    void release() {
        if (ptr_ && (--(*count_)) == 0) {
            delete ptr_;
            delete count_;
        }
    }
    /// @brief 根据传进来的指针，增加引用计数
    /// @param sp 指针
    void add_count(shared_ptr_t<T>& sp) {
        ptr_   = sp.ptr_;
        count_ = sp.count_;
        if (ptr_) {
            ++(*count_);
        }
    }

public:
    /// @brief 构造函数
    /// @param ptr 指针
    shared_ptr_t(T* ptr = nullptr) : ptr_(ptr) {
        if (ptr_) {
            count_ = new int(1);
        }
    }
    /// @brief 拷贝构造函数
    /// @param sp 指针
    shared_ptr_t(shared_ptr_t& sp) { add_count(sp); }
    /// @brief 赋值函数
    /// @param sp 指针
    /// @return shared_ptr_t<T>&
    shared_ptr_t<T>& operator=(shared_ptr_t<T>& sp) {
        if (this != &sp) {
            release();
            add_count(sp);
        }
        return *this;
    }
    /// @brief 析构函数
    ~shared_ptr_t() {
        printf("destruct shared_ptr count: %d, 若解决循环引用的问题最后一行应该显示为1\n", *count_);
        release();
    }
    /// @brief 重载*和->操作符
    /// @return T&
    T&   operator*() { return *ptr_; }
    T*   operator->() { return ptr_; }
    T*   get() { return ptr_; }
    int  use_count() { return *count_; }
    int* count() { return count_; }
};

/// @brief 手写弱指针
/// @tparam T 指针内存类型
template <class T>
class weak_ptr_t {
private:
    int* count_;
    T*   ptr_;

public:
    /// @brief 弱指针构造函数
    /// @param ptr 传入的指针
    weak_ptr_t(T* ptr = nullptr) : count_(nullptr), ptr_(nullptr) {}
    /// @brief 拷贝构造函数，需要搭配手写shared_ptr_t使用，获取共享指针的count指针和ptr指针
    /// @param sp 传入的指针
    weak_ptr_t(shared_ptr_t<T>& sp) : count_(sp.count()), ptr_(sp.get()) {}
    /// @brief 赋值函数
    /// @param sp 传入的指针
    /// @return weak_ptr_t<T>&
    weak_ptr_t<T>& operator=(shared_ptr_t<T>& sp) {
        count_ = sp.count();
        ptr_   = sp.get();
        return *this;
    }
    /// @brief lock函数
    /// @return 若引用计数大于0，返回指针，否则返回空指针
    T* lock() {
        if (count_ && *count_ > 0) {
            return ptr_;
        }
        return nullptr;
    }
    /// @brief 获取引用计数
    /// @return int 引用计数
    int use_count() { return *count_; }
    /// @brief 获取引用计数指针
    /// @return int* 引用计数指针
    int* count() { return count_; }
};

class ListNode {
public:
    int val;
#if false
    shared_ptr_t<ListNode> next;
    shared_ptr_t<ListNode> previous;
#else
    weak_ptr_t<ListNode> next;
    weak_ptr_t<ListNode> previous;
#endif

    ListNode(int x) : val(x), next(nullptr), previous(nullptr) {}
};

int main() {
    // 使用weak_ptr解决shared指针循环引用的问题
    shared_ptr_t<ListNode> p3(new ListNode(800));
    shared_ptr_t<ListNode> p4(new ListNode(800));
    p3->next     = p4;
    p4->previous = p3;
    printf("p3 use count: %d\n", p3.use_count());
    printf("p4 use count: %d\n", p4.use_count());
}