Section 4.3 Relationships between classes

Friend functions (friend keyword)

In order to allow an external function to have access to the private and protected members of a class we have to declare the prototye of the external function that will gain access preceded by the keyword friend within the class declaration that shares its members. In the following example we declare the friend function duplicate:

// friend functions
#include <iostream.h>

class CRectangle {
    int width, height;
  public:
    void set_values (int, int);
    int area (void) {return (width * height);}
    friend CRectangle duplicate (CRectangle);
};

void CRectangle::set_values (int a, int b) {
  width = a;
  height = b;
}

CRectangle duplicate (CRectangle rectparam)
{
  CRectangle rectres;
  rectres.width = rectparam.width*2;
  rectres.height = rectparam.height*2;
  return (rectres);
}

int main () {
  CRectangle rect, rectb;
  rect.set_values (2,3);
  rectb = duplicate (rect);
  cout << rectb.area();
}

From within the duplicate function, that is a friend of CRectangle, we have been able to access the members width and height of different objects of type CRectangle. Notice that neither in the declaration of duplicate() nor in its later use in main() have we considered duplicate as a member of class CRectangle. It isn't.

The friend functions can serve, for example, to conduct operations between two different classes. Generally the use of friend functions is out of an object-oriented programming methodology, so whenever possible it is better to use members of the same class to make the process. Such as in the previous example, it would have been shorter to integrate duplicate() within the class CRectangle.

Friend classes (friend)

// friend class
#include <iostream.h>

class CSquare;

class CRectangle {
    int width, height;
  public:
    int area (void)
      {return (width * height);}
    void convert (CSquare a);
};

class CSquare {
  private:
    int side;
  public:
    void set_side (int a)
      {side=a;}
    friend class CRectangle;
};

void CRectangle::convert (CSquare a) {
  width = a.side;
  height = a.side;
}
  
int main () {
  CSquare sqr;
  CRectangle rect;
  sqr.set_side(4);
  rect.convert(sqr);
  cout << rect.area();
  return 0;
}

In this example we have declared CRectangle as a friend of CSquare so that CRectangle can access the protected and private members of CSquare, more concretely CSquare::side, that defines the square side width.

You may also see something new in the first instruction of the program, that is the empty prototype of class CSquare. This is necessary because within the declaration of CRectangle we refer to CSquare (as a parameter in convert()). The definition of CSquare is included later, so if we did not include a previous definition for CSquare this class would not be visible from within the definition of CRectangle.

Consider that friendships are not corresponded if we do not explicitly specify it. In our CSquare example CRectangle is considered as a class friend, but CRectangle does not do the proper thing with CSquare, so CRectangle can access to the protected and private members of CSquare but not the reverse way. Although nothing prevents us from declaring CSquare as a friend of CRectangle.

Inheritance between classes

This could be represented in the world of classes with a class CPolygon from which we would derive the two referred ones, CRectangle and CTriangle.

Classes derived from others inherit all the visible members of the base class. That means that if a base class includes a member A and we derive it to another class with another member called B, the derived class will contain both A and B.

In order to derive a class from another, we must use the operator : (colon) in the declaration of the derived class in the following way:

class derived_class_name: public base_class_name;

// derived classes
#include <iostream.h>

class CPolygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b;}
  };

class CRectangle: public CPolygon {
  public:
    int area (void)
      { return (width * height); }
  };

class CTriangle: public CPolygon {
  public:
    int area (void)
      { return (width * height / 2); }
  };
  
int main () {
  CRectangle rect;
  CTriangle trgl;
  rect.set_values (4,5);
  trgl.set_values (4,5);
  cout << rect.area() << endl;
  cout << trgl.area() << endl;
  return 0;
}

As you may see, objects of classes CRectangle and CTriangle each contain members of CPolygon, that are: width, height and set_values().

The protected specifier is similar to private, its only difference occurs when deriving classes. When we derive a class, protected members of the base class can be used by other members of the derived class, nevertheless private member cannot. Since we wanted width and height to have the ability to be manipulated by members of the derived classes CRectangle and CTriangle and not only by members of CPolygon, we have used protected access instead of private.

We can summarize the different access types according to whom can access them in the following way:

Access	public	protected	private
members of the same class	yes	yes	yes
members of derived classes	yes	yes	no
not-members	yes	no	no

In our example, the members inherited by CRectangle and CTriangle follow with the same access permission as in the base class CPolygon:

CPolygon::width           // protected access
CRectangle::width         // protected access

CPolygon::set_values()    // public access
CRectangle::set_values()  // public access

class CRectangle: public CPolygon;

class daughter: protected mother;

The most common use of an inheritance level different from public is private that serves to completely encapsulate the base class, since, in that case, nobody except its own class will be able to access the members of the base class from which it is derived. Anyway, in most cases classes are derived as public.

If no access level is explicitly written private is assumed for classes created with the class keyword and public for those created with struct.

What is inherited from the base class?

• Constructor and destructor
• operator=() member
• friends

If the base class has no default constructor or you want that an overloaded constructor is called when a new derived object is created, you can specify it in each constructor definition of the derived class:

derived_class_name (parameters) : base_class_name (parameters) {}

// constructors and derivated classes
#include <iostream.h>

class mother {
  public:
    mother ()
      { cout << "mother: no parameters\n"; }
    mother (int a)
      { cout << "mother: int parameter\n"; }
};

class daughter : public mother {
  public:
    daughter (int a)
      { cout << "daughter: int parameter\n\n"; }
};

class son : public mother {
  public:
    son (int a) : mother (a)
      { cout << "son: int parameter\n\n"; }
};

int main () {
  daughter cynthia (1);
  son daniel(1);
  
  return 0;
}

Observe the difference between which mother's constructor is called when a new daughter object is created and which when it is a son object. The difference is because the constructor declaration of daughter and son:

daughter (int a)          // nothing specified: call default constructor
son (int a) : mother (a)  // constructor specified: call this one

Multiple inheritance

class CRectangle: public CPolygon, public COutput {
class CTriangle: public CPolygon, public COutput {

// multiple inheritance
#include <iostream.h>

class CPolygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b;}
  };

class COutput {
  public:
    void output (int i);
  };

void COutput::output (int i) {
  cout << i << endl;
  }

class CRectangle: public CPolygon, public COutput {
  public:
    int area (void)
      { return (width * height); }
  };

class CTriangle: public CPolygon, public COutput {
  public:
    int area (void)
      { return (width * height / 2); }
  };
  
int main () {
  CRectangle rect;
  CTriangle trgl;
  rect.set_values (4,5);
  trgl.set_values (4,5);
  rect.output (rect.area());
  trgl.output (trgl.area());
  return 0;
}

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