What is the necessity of pointer in this piece of C++ code? - c++

Question

Note - I have searched a lot on pointers and references for C++.
I don't seem to understand them in THIS particular scenario. Hence posting it here!
The following is CORRECT code. It is working. I wrote this for an online C++ practice problem. A part of the code was given to me initially.
I don't understand why an array of Person objects is being created in the main function with a *per[n] as shown below:
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
class Person {
string name;
int age;
public:
Person(){
name = "";
}
virtual void putdata() = 0;
virtual void getdata() = 0;
};
class Professor: public Person {
int publications, cur_id;
string name;
int age;
public:
static int professorCount;
Professor(){
name = "";
age = 0;
publications = 0;
cur_id = professorCount + 1;
professorCount++;
}
void getdata(){
cin >> name >> age >> publications;
}
void putdata(){
cout << name << " " << age << " " << publications << " " << cur_id << endl;
}
};
class Student: public Person {
int marks[6];
int cur_id;
string name;
int age;
public:
static int studentCount;
Student(){
name = "";
age = 0;
cur_id = studentCount + 1;
studentCount++;
}
void getdata(){
cin >> name >> age >> marks[0] >> marks[1] >> marks[2] >> marks[3] >> marks[4] >> marks[5];
}
void putdata(){
cout << name << " " << age << " " << marks[0] + marks[1] + marks[2] + marks[3] + marks[4] + marks[5] << " " << cur_id << endl;
}
};
int Professor::professorCount = 0;
int Student::studentCount = 0;
In this main function below, an array of Person objects is being created, but it is given a * in the beginning. How is it working?
int main(){
int n, val;
cin>>n; //The number of objects that is going to be created.
Person *per[n]; // THIS ONE RIGHT HERE! THIS ONE!
for(int i = 0;i < n;i++){
cin>>val;
if(val == 1){
// If val is 1 current object is of type Professor
per[i] = new Professor;
}
else per[i] = new Student; // Else the current object is of type Student
per[i]->getdata(); // Get the data from the user.
}
for(int i=0;i<n;i++)
per[i]->putdata(); // Print the required output for each object.
return 0;
}

Answer 1

I don't understand why an array of Person objects is being created in the main function with a *per[n] as shown below
The purpose of storing a pointer is to support virtual polymorphism (abstract classes like Person cannot be instantiated). A smart pointer serves that as well, but takes care about the correct dynamic memory management.
There's no need to use raw pointers or raw arrays in c++ at all. That code doesn't give a good example of "best practices".
At the main() function
Person *per[n]; // Note that VLA's aren't standard c++ syntax
should be replaced with
std::vector<std::unique_ptr<Person>> per(n);
and the loop accordingly
for(int i = 0;i < n;i++){
cin>>val;
if(val == 1){
// If val is 1 current object is of type Professor
per[i] = std::make_unique<Professor>();
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
}
// Else the current object is of type Student
else per[i] = std::make_unique<Student>();
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
per[i]->getdata(); // Get the data from the user.
}
Also
int marks[6];
should be replaced with
std::array<int,6> marks;
std::array is way more convenient and less error prone when passed as function parameter, etc.

Answer 2

You're creating an array of pointers to Person objects. That's how assignments like per[i] = new Professor; can work - new Professor returns a pointer to a Professor object, so you need an array of pointers to store that.