#ifndef POLYGON_HPP #define POLYGON_HPP class polygon { protected: double side_length_; public: polygon() : side_length_(0) {} virtual ~polygon() {} void set_side_length(double side_length) { side_length_ = side_length; } virtual double area() const = 0; }; // the types of the class factories typedef polygon* create_t(); typedef void destroy_t(polygon*); #endif