Question

I have an array of objects which all derive from the class BaseStudent.

BaseStudent**studentlist = new BaseStudent*[atoi(listSize.c_str())];

That array is populated with either derived Math, English or History objects. I’m now trying to print out specific data from each object in the array and output it to a file.

for (int j=0; j<atoi(listSize.c_str()); j++){
    if(studentlist[j]->getMT() == ENGLISH){
        output << studentlist[j]->GetFN()<<" "<<studentlist[j]->GetLN();
        output << right << setw(42) << studentlist[j]->GetFinal(); // this is an English public function but I can't call this.
    }
}

I need to be able to access the derived classes private member data from the array of objects.

Here’s my header. As you can see I have a setter and getter for every protected member data.

#include <iostream>
#include <string>

using namespace std;

#ifndef BASESTUDENT_H
#define BASESTUDENT_H

enum MajorType {ENGLISH, HISTORY, MATH};
// *********************************************************************
// Base class. All other classes (Enlish, History, Math) inherit from 
// this class.
// *********************************************************************
class BaseStudent
{
public: 
    BaseStudent();
    BaseStudent(string fn, string ln, string m);
    string GetFN(){return firstName;}
    string GetLN(){return lastName;}
    MajorType getMT(){return course;}
    void SetFN(string fn){firstName = fn;}
    void SetLN(string ln){lastName = ln;}
    void SetMT(string m);

protected:
    string firstName;
    string lastName;
    MajorType course;

}; // End Base class

// *********************************************************************
// Enlish class.
// *********************************************************************

class English: public BaseStudent
{
public:
    English(string fn, string ln, string m, double a, double p, double mt, double f);
    double FinalAverage();
    double GetAttendance(){return attendance;}
    double GetProject(){return project;}
    double GetMidterm(){return midterm;}
    double GetFinal(){return final;}
    double GetFinalAverage(){return finalAverage;}
    void SetAttendance(double a){attendance = a;}
    void SetProject(double p){project = p;}
    void SetMidterm(double m){midterm = m;}
    void SetFinal(double f){final = f;}
    void SetFinalAverage(double fa){finalAverage = fa;}

protected: 
    double attendance;
    double project;
    double midterm;
    double final;
    double finalAverage;

}; // End English class

// *********************************************************************
// History class.
// *********************************************************************

class History: public BaseStudent 
{
public:
    History(string fn, string ln, string m, double t, double mt, double f);
    double FinalAverage();
    double GetTermPaper(){return termPaper;}
    double GetMidterm(){return midterm;}
    double GetFinalExam(){return finalExam;}
    double GetFinalAverage(){return finalAverage;}
    double FinalExam(){return finalExam;}
    void SetTermPaper(double t){termPaper = t;}
    void SetMidterm(double m){midterm = m;}
    void SetFinalExam(double f){finalExam = f;} 
    void SetFinalAverage(double fa){finalAverage = fa;}

protected:
    double termPaper;
    double midterm;
    double finalExam;
    double finalAverage;


}; // End History class.

// *********************************************************************
// Math class.
// *********************************************************************

class Math: public BaseStudent
{
public:
    Math(string fn, string ln, string m, double q1, double q2, double q3,
        double q4, double q, double t1, double t2, double f);
    double FinalAverage();
    double GetQuiz1(){return quiz1;}
    double GetQuiz2(){return quiz2;}
    double GetQuiz3(){return quiz3;}
    double GetQuiz4(){return quiz4;}
    double GetQuiz5(){return quiz5;}
    double GetFinalExam(){return finalExam;}
    double GetTest1(){return test1;}
    double GetTest2(){return test2;}
    double GetQuizAverage(){return quizAverage;}
    double GetFinalAverage(){return finalAverage;}
    void SetQuiz1(double q){quiz1 = q;}
    void SetQuiz2(double q){quiz2 = q;}
    void SetQuiz3(double q){quiz3 = q;}
    void SetQuiz4(double q){quiz4 = q;}
    void SetQuiz5(double q){quiz5 = q;}
    void SetTest1(double q){test1 = q;}
    void SetTest2(double q){test2 = q;}
    void SetFinalExam(double q){finalExam = q;}
    void SetQuizAverage();
    void SetFinalAverage(double fa){finalAverage = fa;}

protected:
    double quiz1;
    double quiz2;
    double quiz3;
    double quiz4;
    double quiz5;
    double test1;
    double test2;
    double finalExam;
    double quizAverage;
    double finalAverage;

}; // End Math class.

#endif

Do I need some sort of implementation of virtual functions?

Here’s my driver so far:

#include<iostream>
#include<fstream>
#include<string>
#include<iomanip>
#include"basestudent.h"

using namespace std;

int main(void)
{
    string listSize;

    string fileIn = "";
    string fileOut = "";
    string firstname ="";
    string lastname ="";
    string major = "";
    string eolDummy;
    int mQuiz1, mQuiz2, mQuiz3, mQuiz4, mQuiz5, mTest1, mTest2, mFinalExam;
    int eAttendance, eProject, eMidterm, eFinalExam;
    int hTermPaper, hMidterm, hFinalExam;

    ifstream input;
    ofstream output;
    do{
        input.clear();

        cout << "Please enter the filename: ";
        cin >> fileIn;
        cout << "Please enter an output name: ";
        cin >> fileOut;

        input.open(fileIn);
        if (!input)
            cout << "Invalid file, please enter again." << endl;
    } while(!input);

    input >> listSize;
    BaseStudent**studentlist = new BaseStudent*[atoi(listSize.c_str())];
    int i = 0;
    while (!input.eof())
    {
        getline(input, lastname, ',');
        getline(input, firstname, '\n');

        input >> major;

        if (major == "Math") {
            input >>mQuiz1>>mQuiz2>>mQuiz3>>mQuiz4>>mQuiz5>>mTest1>>mTest2
                  >>mFinalExam>>eolDummy;
            // Math Constructor call
            // Array += object
            studentlist[i] = new Math(firstname,lastname,major,mQuiz1,mQuiz2,mQuiz3,mQuiz4,mQuiz5,
                                      mTest1,mTest2,mFinalExam);
        }
        else if (major == "History"){
            input >>hTermPaper>>hMidterm>>hFinalExam>>eolDummy;
            // History Constructor call
            // Array += object
            studentlist[i] = new History(firstname,lastname,major,hTermPaper,hMidterm,hFinalExam);
        }
        else if(major == "English"){
            input >>eAttendance>>eProject>>eMidterm>>eFinalExam>>eolDummy;
            // English Constructor call
            // Array += object
            studentlist[i] = new English(firstname,lastname,major,eAttendance,eProject,eMidterm,eFinalExam);
        }
        i++;
    }

    output.open(fileOut);
    output << "Student Grade Summary" << endl;
    output << "---------------------" << endl << endl;
    output << "ENGLISH CLASS "<<endl<<endl;
    output << "Student                                   Final   Final   Letter"<<endl;
    output << "Name                                      Exam    Avg     Grade"<<endl;
    output << "----------------------------------------------------------------"<<endl;
    for (int j=0; j<atoi(listSize.c_str()); j++){
        if(studentlist[j]->getMT() == ENGLISH){
            output << studentlist[j]->GetFN()<<" "<<studentlist[j]->GetLN();

            output << right << setw(42) << studentlist[j]->



    input.close();
    output.close();
    return 0;
}

Answer 1

When you take the pointer from your array, you need to cast it using dynamic_cast to the appropriate class

e.g.

BaseStudent *p = somearray[0];

if ( English* pEnglish = dynamic_cast<English*>(p) )
{
   // call the methods
   cout << p->FinalAverage();
   ...
}
else if ( History* pHistory = dynamic_cast<History*>(p) )
{
   // call the methods
}
else if ( Math* pMath = dynamic_cast<Math*>(p) )
{
   // call the methods
}

btw use a vector instead of a raw array, it is more convenient and safer.

std::vector<BaseStudent*> yourvector;