text: #ifndef FIGURE_H
  #define FIGURE_H
  const double PI = 3.14159; // 圆周率常量
  
  class FIGURE
  {
  public:
      void set_size(double x, double y = 0);
      virtual double get_area() = 0; // get_area()被声明为纯虚函数
  protected:
      double x_size, y_size;
  };
  
  #endif
  