Question

I’m writing code to store exam results for different degrees (physics & chemistry currently).

I have an abstract base class student which is as follows:

class student{
public:
    virtual ~student(){}
    //virtual function to add course + mark
    virtual void addresult(std::string cName,int cCode,double cMark)=0;
    //virtual function to print data
    virtual void printinfo()=0;     //to screen
    virtual void outputinfo(std::string outputfilename)=0;      //to file
};

I then have a derived class for physics (and will have a similar one for chemistry):

class physics : public student{
protected:
    std::string studentname;
    int studentID;
    std::map<int,course> courses; //map to store course marks
public:
    //constructors/destructors
    void addresult(std::string cName,int cCode,double cMark){course temp(cName,cCode,cMark);courses[cCode]= temp;}

    void printinfo(){
        //function to output information to screen      
    }

    void outputinfo(std::string outputfilename){
        //function to write student profile to file
    }
};    

In my main, I would then like to have a map that can store all students within it (physics and chemistry). The student ID as the key with a base class pointer to either a physics or chem. student is, I’m guessing, the way to go.

I tried the following code:

map<int,student*> allstudents;
physics S1(sName,sID);
physics* S2 = &S1;
allstudents.insert(pair<int,student*>(sID,S2));

but this didn’t work. I think I’m getting slightly confused with what should be pointing to what. Can you even do this with maps?
Should there also be any ‘clearing up’ required if I store the info. this way?
Thanks for your help.

Answer 1

If you use a pointer for more than a mere second, you should not create the object on the stack and then point to it! It will be gone as soon as the next } shows up, and your pointer will be invalid!

Use physics* S2 = new physics(sName, sID); instead. Use delete on all pointers in your map (iterator comes handy here) to clean up!