Object Array of Derived Objects - c++

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;