Protected

• There is a third type of access you may declare your member functions and data to be. The presence of the keyword label protected: in a class definition causes all further declarations to be private: and invisible to the outside world. but public: and useable to any derived classes.

• Unless you really have a good reason for not wanting derived classes to use something, most private: data really should be protected:. If the side lengths in the triangle class had been declared protected, then the implementation of rightTriangle would have been much easier:

  // rightTriangle.h
  
  class rightTriangle : public triangle
  {
     public:
          rightTriangle();
          rightTriangle(double side1, double side2);
  
          void setTriangle(double side1, double side2);
  };
  

  ---

  // rightTriangle.C
  
  #include <math.h>
  
  rightTriangle::rightTriangle()
  {
    s1 = 0; s2 = 0; s3 = 0;
  }
  
  rightTriangle::rightTriangle(double side1, double side2)
  {
    s1 = side1; s2 = side2;
    s3 = sqrt( s1 * s1 + s2 * s2 );
  }
  
  void rightTriangle::setTriangle(double side1, double side2)
  {
    s1 = side1; s2 = side2;
    s3 = sqrt( s1 * s1 + s2 * s2 );
  }
  

  Note that the public interface of rightTriangle stayed the same!