c++ - 无法访问派生类中的 protected 类成员

Question

错误是:

d_start is a protected member of CourseActivity
duration is a protected member of CourseActivity
location is a protected member of CourseActivity

class CourseActivity{

protected:
    StartTime* d_start;
    double duration;
    std::string location;

public:
    CourseActivity() = default;
    CourseActivity(const StartTime* _start, double _duration,
                   const std::string_location);
    void reschedule(StartTime* _newStart);
    void print() const;

}; 



class Lecture: public CourseActivity{
    std::string topic;
    bool deflt = false; //indicate which constructor was used.
                        //false = 1st. true = 2nd

public:
    Lecture(const StartTime* _start, double _duration,
            const std::string location, const std::string& _topic);
    Lecture(const CourseActivity& _oActivity, const std::string& topic );
    void print();
};

// ERROR
Lecture::Lecture(const CourseActivity& _oActivity, const std::string& _topic)
: CourseActivity(_oActivity.d_start,_oActivity.duration,_oActivity.location){
    topic = _topic;
    deflt = true;
}
// ERROR 

Answer 1

您正在将 CourseActivity 的实例传递给函数 Lecture::Lecture。即使 CourseActivity 确实是 Lecture 的基类，您也无法从外部访问 protected 类成员(如 _oActivity.duration)，即使您正在操作的对象属于派生类型。

为避免您的特殊问题，您可以在基类中创建此构造函数

CourseActivity::CourseActivity(const CourseActivity &_oActivity)

并调用它

Lecture::Lecture(const CourseActivity& _oActivity, const std::string& _topic)
    : CourseActivity(_oActivity)

在派生类中。在基类中，您可以访问 protected 成员，而在派生类中则不允许这样做。