C++ Virtual function abstract class definition

#include <iostream>
#include  <string>

class Pool{//  w ww  . j  a va2 s .  c om
protected:
  double length {1.0};
  double width {1.0};
  double height {1.0};

  Pool(double lv, double wv, double hv) : length {lv}, width {wv}, height {hv}{}

public:
  virtual double volume() const = 0;   // Function to calculate the volume
};
using  std::string;

class  Carton : public Pool
{
private:
  string material;

public:
  // Constructor  explicitly calling the  base constructor
  Carton(double lv, double wv, double hv, string str = "material") : Pool {lv, wv, hv}
  {
    material = str;
  }

  // Function  to  calculate the  volume of  a Carton object
  double volume() const override
  {
    double vol {(length - 0.5)*(width - 0.5)*(height - 0.5)};
    return vol > 0.0 ? vol : 0.0;
  }
};

class  ToughPack : public Pool{
public:
  // Constructor
  ToughPack(double lv, double wv, double hv) : Pool {lv, wv, hv} {}

  // Function  to  calculate volume of  a ToughPack  allowing 15%  for  packing
  double volume()  const override { return 0.85*length*width*height; }
};

int main()
{
  ToughPack hardcase {20.0, 30.0, 40.0};         // A derived pool - same size
  Carton carton {20.0, 30.0, 40.0, "plastic"};   // A different derived pool

  Pool*pPool {&hardcase};
  std::cout << "hardcase volume is " << pPool->volume() << std::endl;
  pPool = &carton;
  std::cout << "carton volume is " << pPool->volume() << std::endl;
}