在c++编译过程中，“有虚拟方法……但非虚拟析构函数”的警告是什么意思?

#include <iostream>
using namespace std;

class CPolygon {
  protected:
    int width, height;
  public:
    virtual int area ()
      { return (0); }
  };

class CRectangle: public CPolygon {
  public:
    int area () { return (width * height); }
  };
#include <iostream>
using namespace std;

class