c ++多态/纯虚函数

Question

I have a class called course. Course has a pointer to a base class called Assessment.

class Course{
    char* courseName;
    float fee;

public:
    Assessment* assessment;
    Course();
    Course(Course&);
    Course(char*, float, Assessment *);
    ~Course();
    friend ostream& operator <<(ostream& os, Course&);
};

Strickly for testing purposes, in the constructor i'm assigning the Assessment pointer to an ExamAssessment(a sub-class)

Course::Course(){
    courseName = "something";
    fee = 0;
    assessment = &ExamAssessment(45);
}

My driver code looks like this

Course t = Course();
cout << *(t.assessment);

In assessment, Report is a pure virtual

virtual void Report() = 0;

The problem is when I cout the assessment, in the assessment overloaded operator '<<', I'm calling Report()

ostream& operator<<(ostream& os, Assessment& assessment){
    assessment.Report();
    os << "Course Grade: "<< assessment.grade << endl;
    return os;
}

But the Report is calling 'Assessments' Report instead of ExamAssessments Report. Not sure where I'm going wrong with this(I'm fairly new to C++ inheritance)

Answer 1

In your Course::Course() you should never do that

 assessment = &ExamAssessment(45);

You're taking the address of a temporary. That will be invalid as soon as you're out of the constructor scope. And I think the compiler warned you ! That explain your crash unexpected behaviour when you're accessing assessment.

Do:

 assessment = new ExamAssessment(45);

And as @Michael K. Sondej said delete it in the destructor. Or use std::unique_ptr .