An Example of the Decorator Pattern (p 175)

Intent

In this example we are decorating patients with problems. We decorate a patient called "Clyde" who is hung over, smashed his knee, and writes buggy code. "Bonnie" is a sane patient who is also hung over.

NOTE: Please see below for a commentary on this example by Design Patterns author Ralph Johnson.

How "Clyde" Explains his Problems

Illness decorations on object "Clyde" and the recusive calls to his ExplainProblem() method.

Example Program illustrating a Decorator Pattern for Patients

Structure

Code

#include <iostream.h>
/*
  Example of the Decorator Pattern (p. 175)
  J. Kerievsky, October 1995
*/

class Person
{
  public:
    virtual void Explain_Problems() {}
  protected:
    Person() { }
};


class Patient : public Person
{
 public:
   Patient() : Person() {}
   void Explain_Problems() {}
};

class PatientDecorator : public Person
{
 public:
   PatientDecorator(Person* p) : Person() { _NextPatientDecorator = p; }
   void Explain_Problems() { _NextPatientDecorator->Explain_Problems(); }
 private:
   Person* _NextPatientDecorator;
};


class SanePatient : public PatientDecorator
{
 public:
   SanePatient(Person* p) : PatientDecorator(p) { }
   void Explain_Problems() {
     cout << "\tSane." << endl;
     PatientDecorator::Explain_Problems(); }
};


class HungOverPatient : public PatientDecorator
{
 public:
   HungOverPatient(Person* p, int num_drinks = 5) : PatientDecorator(p)
      { _drinks_drunk = num_drinks; }
   void Explain_Problems() {
      Explain();
      PatientDecorator::Explain_Problems(); }
 protected:
   void Explain() { cout << "\tHungover";  Burp();  }
   void Burp() { if (_drinks_drunk > 5) cout << " (Buuuuurrppp)";  cout << "." << endl; }
 private:
   int _drinks_drunk;
};


class BugRiddenPatient : public PatientDecorator
{
 public:
   BugRiddenPatient(Person* p) : PatientDecorator(p) {}
   void Explain_Problems() {
      cout << "\tA buggy software developer." << endl;
      PatientDecorator::Explain_Problems(); }
};


class SmashedKneePatient : public PatientDecorator
{
 public:
   SmashedKneePatient(Person* p) : PatientDecorator(p) {}
   void Explain_Problems() {
      cout << "\tNursing a smashed knee." << endl;
      PatientDecorator::Explain_Problems(); }
};


void TellMeWhatIsWrong(Person* p, char* n);

void main(void)
{
  // mix and match illness decorations for a patient
  Person* Clyde = new HungOverPatient(
                      new SmashedKneePatient(
                        new BugRiddenPatient(
                          new Patient())),6);
  TellMeWhatIsWrong(Clyde, "Clyde");


  Person* Bonnie = new SanePatient(
                      new HungOverPatient(
                        new Patient()));
  TellMeWhatIsWrong(Bonnie, "Bonnie");
}


void TellMeWhatIsWrong(Person* p, char* n)
{
   cout << n << " is " << endl;
   p->Explain_Problems();
   cout << endl;
}


Output of Example Program
Clyde is
        Hungover (Buuuuurrppp).
        Nursing a smashed knee.
        A buggy software developer.

Bonnie is
        Sane.
        Hungover.