How to work with interfaces?

Question

C++ / C#

I have a code in which I am trying to implement the GJK algorithm (algorithm for finding the intersection of two shapes).
The main gjk.h file looks like this:

#include "Vector.h"
#include "Collision.h"
#include "Simplex.h"
Vector support(IShape a, IShape b, Vector direction){
  Vector aFar = a.FarthestPointInDirection(direction);
  Vector bFar = b.FarthestPointInDirection(direction);
  return *aFar.Sub(bFar);
}
Collision Calculate(IShape a,IShape b){
  Simplex simplex = *(new Simplex());
  Vector direction = *new Vector(0,1);
  Vector initSupportPoint = support(a,b,direction);
  simplex.Add(initSupportPoint);
  direction = *direction.Invert();
  while(direction.null){
    Vector supportPoint=support(a,b,direction);
    if (supportPoint.Dot(direction)<=0){
      return *new Collision(a,b,true);
    }
    simplex.Add(supportPoint);
    direction=simplex.CalculateDirection();
  }
  return *new Collision(a,b,false);
}

Here IShape is used, which is an interface. It is defined in its IShape.h file

#ifndef ISHAPE_H
#define ISHAPE_H
class IShape
{
  public:
    virtual Vector FarthestPointInDirection(Vector direction);
};

#endif

There are two classes that implement this interface. They are in their files:
polygon.h

#ifndef POLYGON_H
#define POLYGON_H


class polygon : public IShape{
  public:
    vector<Vector> points;
    polygon(vector<Vector> points){
      this->points=points;
    }
    virtual Vector FarthestPointInDirection(Vector direction){
      int farthestDistance=-2147483647;
      Vector farthestPoint = *new Vector(0,0);
      for (int point = 0;point<this->points.size();point++){
        int distanceInDirection = this->points[point].Dot(direction);
        if (distanceInDirection>farthestDistance){
          farthestPoint=this->points[point];
          farthestDistance=distanceInDirection;
        }
        
      }
      return farthestPoint;
    }
};


#endif

circle.h:

#ifndef CIRCLE_H
#define CIRCLE_H


class Circle : public IShape
{
  public:
    Vector center;
    int radius;
    
    Circle(int radius,Vector center)
    {
      this->radius=radius;
      this->center=center;
    }
    virtual Vector FarthestPointInDirection(Vector direction){
      float angle = atan2(direction.y,direction.x);
      return *new Vector(this->center.x+(this->radius*cos(angle)),
      this->center.y+(this->radius*sin(angle)));
    }
};

#endif

There is nothing in the main.cpp file except That is, no functions are called. The compiler generates the following error: How do I use interfaces correctly, and what am I doing wrong?
#include "gjk.h"

Answer 1

You have some trouble understanding pointers.
You need to pass a pointer to IShape to the function if you want to work with the inheritance chain.
But structures like

Simplex simplex = *(new Simplex());

- just a fierce game.

Answer 2

Approximately so

class IShape
{
  public:
    virtual Vector FarthestPointInDirection(Vector direction) = 0; // pure virtual function
};

class Polygon : public IShape{
...
Vector FarthestPointInDirection(Vector direction) override{
....
}
};

Vector support(IShape& a, IShape& b, Vector direction){
 Vector aFar = a.FarthestPointInDirection(direction);
  Vector bFar = b.FarthestPointInDirection(direction);
  return *aFar.Sub(bFar);
}