【面试总结】C++手写智能指针shared_ptr

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shared_ptr

代码

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

namespace smart_pointer
{

    template <typename T>
    struct defaultDeleter
    {
        void operator()(const T *ptr)
        {
            if (ptr)
            {
                delete ptr;
                ptr = nullptr;
            }
        }
    };

    template <typename T, typename Deleter = defaultDeleter<T>>
    class shared_ptr
    {
    public:
        shared_ptr();
        shared_ptr(T *ptr);
        shared_ptr(const shared_ptr &sp); // 拷贝构造
        shared_ptr(shared_ptr &&sp);      // 移动构造
        ~shared_ptr();

        T *operator->() const;
        T &operator*() const;
        operator bool() const;

        shared_ptr &operator=(const shared_ptr &sp);
        shared_ptr &operator=(shared_ptr &&sp);

        T *get() const;
        void reset(T *ptr);
        void swap(shared_ptr &sp);
        int count();

    private:
        atomic<int> *use_count;
        T *ptr;
    };

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr()
    {
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(T *_ptr)
        : ptr(_ptr), use_count(new atomic<int>(1))
    {
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(const shared_ptr &sp)
        : ptr(sp.ptr), use_count(sp.use_count)
    {
        ++*use_count;
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(shared_ptr &&sp)
    {
        std::swap(ptr, sp.ptr);
        std::swap(use_count, sp.use_count);
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::~shared_ptr()
    {
        if (ptr)
        {
            --*use_count;
            if (*use_count <= 0)
            {
                Deleter()(ptr);
                delete use_count;
                cout << "shared_ptr dctor" << endl;
            }
        }
    }

    template <typename T, typename Deleter>
    T *shared_ptr<T, Deleter>::operator->() const
    {
        return ptr;
    }

    template <typename T, typename Deleter>
    T &shared_ptr<T, Deleter>::operator*() const
    {
        return *ptr;
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::operator bool() const
    {
        return ptr != nullptr;
    }

    /* 异常安全赋值 */
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter> &shared_ptr<T, Deleter>::operator=(const shared_ptr<T, Deleter> &sp)
    {
#if 1
        // copy and swap
        shared_ptr<T, Deleter> tmp(sp);
        swap(tmp);
        return *this;
#else
        // 自我赋值
        if (sp.ptr == ptr)
            return *this;

        ptr = sp.ptr;
        use_count = sp.use_count;
        ++*use_count;
        return *this;
#endif
    }

    template <typename T, typename Deleter>
    shared_ptr<T, Deleter> &shared_ptr<T, Deleter>::operator=(shared_ptr<T, Deleter> &&sp)
    {
        std::swap(ptr, sp.ptr);
        std::swap(use_count, sp.use_count);
        return *this;
    }

    template <typename T, typename Deleter>
    T *shared_ptr<T, Deleter>::get() const
    {
        return ptr;
    }

    template <typename T, typename Deleter>
    void shared_ptr<T, Deleter>::reset(T *_ptr)
    {
        shared_ptr<T, Deleter>().swap(*this);

        ptr = _ptr;
        use_count = new atomic<int>(1);
    }

    template <typename T, typename Deleter>
    void shared_ptr<T, Deleter>::swap(shared_ptr<T, Deleter> &sp)
    {
        std::swap(ptr, sp.ptr);
        std::swap(use_count, sp.use_count);
    }

    template <typename T, typename Deleter>
    int shared_ptr<T, Deleter>::count()
    {
        return *use_count;
    }

}; // namespace smart_pointer

int main(int argc, char *argv[])
{
    {
        smart_pointer::shared_ptr<int> p(new int(12));
        smart_pointer::shared_ptr<int> p1(p);
        *p1 = 2;
        cout << *p << endl;
        cout << p.count() << endl;

        // 异常安全测试
        p = p;
        cout << *p << endl;
        cout << p.count() << endl;
    }

    {
        smart_pointer::shared_ptr<int> p(new int(12));
        p.reset(new int(13));
        cout << *p << endl;

        // 右值赋值测试
        p = smart_pointer::shared_ptr<int>(new int(14));
        cout << *p << endl;

        // 移动构造测试
        smart_pointer::shared_ptr<int> p1(smart_pointer::shared_ptr<int>(new int(15)));
        cout << *p1 << endl;
    }

    return 0;
}

实验结果

简化版

面试时上面代码很难在短时间内写完，可以参考下面的代码

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

namespace smart_pointer
{
    template <typename T>
    struct defaultDeleter
    {
        void operator()(const T *ptr)
        {
            if (ptr)
            {
                delete ptr;
                ptr = nullptr;
            }
        }
    };
    template <typename T, typename Deleter = defaultDeleter<T>>
    class shared_ptr
    {
    public:
        shared_ptr();
        shared_ptr(T *ptr);
        shared_ptr(const shared_ptr &sp);
        shared_ptr(shared_ptr &&sp);
        ~shared_ptr();
        // T *operator->() const;
        T &operator*() const;
        int count();
        // shared_ptr &operator=(const shared_ptr &sp);
        // shared_ptr &operator=(shared_ptr &&sp);
    private:
        atomic<int> *use_count;
        T *ptr;
    };
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr()
    {
    }
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(T *_ptr)
        : ptr(_ptr), use_count(new atomic<int>(1))
    {
    }
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(const shared_ptr &sp)
        : ptr(sp.ptr), use_count(sp.use_count)
    {
        ++*use_count;
    }
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::shared_ptr(shared_ptr &&sp)
    {
        std::swap(ptr, sp.ptr);
        std::swap(use_count, sp.use_count);
    }
    template <typename T, typename Deleter>
    shared_ptr<T, Deleter>::~shared_ptr()
    {
        if (ptr)
        {
            --*use_count;
            if (*use_count <= 0)
            {
                Deleter()(ptr);
                delete use_count;
                cout << "shared_ptr dctor" << endl;
            }
        }
    }

    template <typename T, typename Deleter>
    T &shared_ptr<T, Deleter>::operator*() const
    {
        return *ptr;
    }

    template <typename T, typename Deleter>
    int shared_ptr<T, Deleter>::count()
    {
        return *use_count;
    }
};

int main()
{
    smart_pointer::shared_ptr<int> p(new int(12));
    smart_pointer::shared_ptr<int> p1(p);

    *p = 33;
    cout << *p << endl;
    cout << p.count() << endl;
    return 0;
}

面试实战

智能指针被问到多次，一般都是先简单按照自己的理解解释c++智能指针，然后有些面试官会要求自己手写一遍，如果可以仔细研究此代码，还是可以学到很多知识点的。

手写智能指针在今年的大厂面试中，自己也是遇到了多次，基本上有会要求手写一点，时间大概10分钟左右，所以平时在复习的时候，一定要十分熟练。开始的时候，自己以为看懂了就行了，结果在面试中高压环境下写代码的时候，会犯很多的错误，所以平时一定要自己多练习，规定时间内熟练的写出来。