Can't access protected class member in a derived class

Question

The errors are:

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

possible duplicate of access protected inherited member with pointer to base class — dyp
You can only access the protected methods of the parent from an object that is descending from that parent. Here, oActivity is not a parent of the newly created object, hence its protected members cannot be accessed. — Ashalynd
Or maybe it's a duplicate of this question. Anyway, it's a duplicate. — dyp

Atle Atle · Accepted Answer · 2013-10-14T00:18:24

You are passing an instance of CourseActivity to the function Lecture::Lecture. Even while CourseActivity indeed is the base class of Lecture, you cannot access protected class members from the outside (like _oActivity.duration) even if the object you're operating on is of a derived type.

To avoid your particular problems, you may create this constructor in the base class

CourseActivity::CourseActivity(const CourseActivity &_oActivity)

and call it with

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

in the derived class. In the base class, you can then access the protected members, as opposed to in the derived class, where this is not allowed.

Can't access protected class member in a derived class

1 Answers