why and when should we use an abstract class in c++ with example

Example 1: what is abstract class in c++

//Code by Soumyadeep Ghosh 
//insta : @soumyadepp
//linked in : https://www.linkedin.com/in/soumyadeep-ghosh-90a1951b6/
#include <bits/stdc++.h>

using namespace std;

class person
{
  string p_id;
  public:
  virtual void get_info()=0; //declaring person as abstract class
  virtual void show()=0;
};

class student:public person
{
  string name;
  int roll_no;
  public:
  /*overriding the pure virtual function declared in base class otherwise
    this class will become an abstract one and then objects cannot be created
    for the same*/
    void get_info()
    {
      cout<<"Enter name of the student "<<endl;
      cin>>name;
      cout<<"Enter roll number of the student "<<endl;
      cin>>roll_no;
    }
   void show()
   {
     cout<<"Name : "<<name<<" Roll number: "<<roll_no<<endl;
   }
};

int main()
{
  person *p;
  p=new student;
  p->get_info();
  p->show();
  return 0;
}

Example 2: abstract class in c++

struct Abstract {
    virtual void f() = 0; // pure virtual
}; // "Abstract" is abstract
 
struct Concrete : Abstract {
    void f() override {} // non-pure virtual
    virtual void g();     // non-pure virtual
}; // "Concrete" is non-abstract
 
struct Abstract2 : Concrete {
    void g() override = 0; // pure virtual overrider
}; // "Abstract2" is abstract
 
int main()
{
    // Abstract a; // Error: abstract class
    Concrete b; // OK
    Abstract& a = b; // OK to reference abstract base
    a.f(); // virtual dispatch to Concrete::f()
    // Abstract2 a2; // Error: abstract class (final overrider of g() is pure)
}