accessing an array through pointer - c++

Question

I wanted to create a database like class through OOP. In this class, there are 3 arrays which act as columns in a table.I insert data through insert() function and prints data through printEntries() function. But that function can't access the arrays to retrieve data.
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
using namespace std;
class subject{
public:
int numOfStudents;
string subjectId;
int * marksArray;
int * indexArray;
char * gradeArray;
int index ; // index for inserting data
subject(int students , string subjectName){
numOfStudents = students;
subjectId = subjectName;
this->index =0 ;
//creating and pointing to arrays
int A[numOfStudents]; marksArray = A;
int B[numOfStudents]; indexArray = B;
char C[numOfStudents]; gradeArray = C;
}
void insert(int studentId , int mark , char grade){
indexArray[index] = studentId;
marksArray[index] = mark;
gradeArray[index] = grade;
this->index = this->index +1;
}
int getNumberOfEntries(){
return index ;
}
void printEntries(){
cout<< indexArray[0] << " O" << marksArray[0] << " " << gradeArray[0] << endl;
cout<< indexArray[1] << " OOO" << marksArray[1] << " " << gradeArray[1] << endl;
cout<< indexArray[2] << " OO" << marksArray[2] << " " << gradeArray[2] << endl;
}
};
int main(int argc, char const *argv[]){
subject S(10,"Mathematics");
cout<<S.subjectId<<endl;
cout<<S.numOfStudents<<endl;
S.insert(35,34,'A');
S.insert(33,34,'B');
S.insert(54,34,'C');
S.insert(21,34,'D');
S.insert(14,34,'F');
S.printEntries();
return 0;
}
output is :
Mathematics
10
35 O34 A
0 OOO0

Answer 1

As pointed out by #paddy in the comments to your question, your issue is within the constructor.
//creating and pointing to arrays
int A[numOfStudents]; marksArray = A;
int B[numOfStudents]; indexArray = B;
char C[numOfStudents]; gradeArray = C;
What you are doing is saving the address of the very first element and the rest disappears after you leave the constructor. you are lucky that it's even allowing you to save the first insert and to quote the comment what you are doing is "defined is undefined behavior"
Replacing the pointers with std::vectors and inserting you elements there will be must easier.
class subject
{
public: // It's considered bad practive to have public data members in a class
std::vector<int> marksVector;
std::vector<int> indexVector;
std::vector<char> gradeVector;
// ...
void insert(int studentId , int mark , char grade){
indexVector.push_back(studentId);
marksVector.push_back(mark);
gradeVector.push_back(grade);
// No longer need to worry about index for next insert
}
};