A pure virtual class, notably because of methods that = 0 (also called "pure virtual" methods.) Classes that are pure virtual cannot be instantiated, only subclasses that implement the pure virtual methods can.

Circle.cpp

#include "Circle.h"

const double Circle::PI = 3.14;

Circle::Circle(double radius):
  radius_(radius) {}

double Circle::area() const {
  return _radius * _radius * PI;
}

Circle.h

#include "Shape.h"

class Circle: public Shape {
  public:
    Circle(double radius);
  
    virtual double area() const override;
  
  private:
    double radius_;
    static const double PI;
};

main.cpp

#include "Rectangle.h"
#include "Circle.h"
#include "Shape.h"

#include <iostream>

Shape* give_me_a_shape(int select) {
  if (select >= 0) {
    return new Rectangle(select + 1.0, select + 1.0);
  }
  
  return new Circle(-select);
}

int main(int argv, const char* argv[]) {
  //Shape s1; // Can't be created because pure virtual, compile error
  
  Rectangle r1;
  
  // Compiler directly inserts call to Rectangle::area
  std::cout << r1.area() << std::endl;
  
  Shape& s2 = r1; // Ok
  
  // ??
  std::cout << s2.area() << std::endl;
  
  Shape* s3 = give_me_a_shape(argv - 3);
  
  // ??
  std::cout << s3->area() << std::endl;
  
  delete s3;
  
  return 0;
}

Rectangle.cpp

#include "Rectangle.h"

Rectangle::Rectangle(double length, double width):
  length_(length),
  width_(width) {}

double Rectangle::area() const {
  return length_ * width_;
}

Rectangle.h

#include "Shape.h"

class Rectangle: public Shape {
  public:
    Rectangle(double length, double width);
    virtual double area() const override;
  
  private:
    double length_;
    double width_;
};

Shape.h

class Shape {
  public:
    // Remember to make your destructors virtual kids
    virtual ~Shape();
  
    virtual double area() const = 0;
};