Question

我正在使用我的代码，我遇到了这个问题。我想在单个向量中放入3个不同的类数据，因此我创建了基类向量。它看起来像这样：

std::vector <Video*> vid_list;

当我读取文件时，我使用其中一个派生类构造函数推送此向量中的数据：

vid_list.push_back(new Official_Music_Video(video_title, video_time,
                Date(y,m,d), num_of_views));

所以我想在屏幕上显示这些数据，但我的基类只有

的方法

std::string title;
double time;
Date date_of_release;

但不适用于int number_of_views。我听说如果我想使用派生类getter方法我必须使用upcasting所以我试图做到这个

void views (Video& v) {
    v.get_views();
}

然后在我的显示方法

void Official_Music_Video::display_info(std::vector <Video*>& vid_list){
    Official_Music_Video ofc;
    for (unsigned int i = 0; i < vid_list.size(); i++){
        cout << vid_list[i]->get_title() << " " << vid_list[i]->get_time()
        << " " << vid_list[i]->write_year_to_file() << "-" <<
        vid_list[i]->write_month_to_file() << "-" << vid_list[i]- 
        >write_day_to_file () << endl;
        cout << "This video viewed " << vid_list[i].views(ofc) << " times" 
        << endl;
   } 
}

但它没有用。你能帮我理解我犯错的地方和错误吗？

Answer 1

如果所有三个派生类都有“number_of_views”方法，那么也将它放在基类中，问题就解决了。如果没有，那么你的代码是荒谬的。当你将“Unofficial_Music_Video”放入向量并在其上运行循环时，你会发生什么？

如果您确实需要在类中添加per-type行为，请考虑您实际想要完成的任务。您是否需要视频用户才能查询“number_of_views”，尽管所有子类型都不存在该字段？也许您只想访问人类可读的统计数据描述？如果是这种情况，最好在Video中添加“virtual std :: string StatsSummary（）”方法，并让子类根据需要实现它。

Answer 2

您可能希望将某些实现移动到基类左右，但这可以帮助您找到解决方案：

#include <vector>
#include <string>
#include <iostream>

class Date // for demo only
{
public:
    Date(int y, int, int m, int d)
        : y(y) , m(m), d(d) {}
    int y;
    int m;
    int d;
};

class Video
{
public:

    virtual std::string get_title() = 0;
    virtual int get_time() = 0;
    virtual int write_year_to_file() = 0;
    virtual int write_month_to_file() = 0;
    virtual int write_day_to_file () = 0;
    virtual ~Video(){}
    virtual void display_info()
    {
        std::cout << get_title()
            << " " << get_time()
            << " " << write_year_to_file()
            << "-" << write_month_to_file()
            << "-" << write_day_to_file ()
            << std::endl;
    }
};


class VideoWithViewCount
    : public Video
{
public:
    virtual int views() = 0;

        virtual void display_info()
    {
        Video::display_info();
        std::cout << "This video viewed " << views() << " times"  << std::endl;
    }

};


class Official_Music_Video : public VideoWithViewCount
{
public:
    Official_Music_Video(std::string _video_title, int _video_time,  Date _date, int _Num_of_views)
        : d(_date)
        , time(_video_time)
        , Num_of_views(_Num_of_views)
        , video_title(_video_title)
    {}

    virtual int get_time() override {return time; };
    virtual int write_year_to_file() override { return d.y; } // better call them get...
    virtual int write_month_to_file() override {return d.m; }
    virtual int write_day_to_file () override { return d.d; }
    virtual int views() override {return Num_of_views; };
private:
    Date d;
    int time;
    int Num_of_views;
    std::string video_title;
};

class Other_Video : public Video
{
public:
    Other_Video(std::string _video_title, int _video_time,  Date _date)
        : d(_date)
        , time(_video_time)
        , video_title(_video_title)
    {}

    virtual int get_time() override {return time; };
    virtual int write_year_to_file() override { return d.y; }
    virtual int write_month_to_file() override {return d.m; }
    virtual int write_day_to_file () override { return d.d; }
private:
    Date d;
    int time;
    std::string video_title;
};

int main(int,char**)
{
    std::vector <Video*> vid_list;
    // could also use unique_ptr or shared_ptr  from #include <memory> in vector as follows:
    // std::vector <unique_ptr<Video> > vid_list;
    // std::vector <shared_ptr<Video> > vid_list;

    // fill video vector
    // vid_list.push_back(new Official_Music_Video(video_title, video_time,  Date(y,m,d), num_of_views));

    for (auto v : vid_list) // when using unique_ptr remember to use "const auto&" instead of "auto"
    {
        v->display_info();
    }

    return 0;
}

向上转换与c ++中的向量

2 个答案: