/ Design Patterns / Creational patterns / Prototype

Prototype Design Pattern in C++: Before and after

Before

The architect has done an admirable job of decoupling the client from Stooge concrete derived classes, and, exercising polymorphism. But there remains coupling where instances are actually created. 

class Stooge
{
  public:
    virtual void slap_stick() = 0;
};

class Larry: public Stooge
{
  public:
    void slap_stick()
    {
        cout << "Larry: poke eyes\n";
    }
};
class Moe: public Stooge
{
  public:
    void slap_stick()
    {
        cout << "Moe: slap head\n";
    }
};
class Curly: public Stooge
{
  public:
    void slap_stick()
    {
        cout << "Curly: suffer abuse\n";
    }
};

int main()
{
  vector roles;
  int choice;

  while (true)
  {
    cout << "Larry(1) Moe(2) Curly(3) Go(0): ";
    cin >> choice;
    if (choice == 0)
      break;
    else if (choice == 1)
      roles.push_back(new Larry);
    else if (choice == 2)
      roles.push_back(new Moe);
    else
      roles.push_back(new Curly);
  }
  for (int i = 0; i < roles.size(); i++)
    roles[i]->slap_stick();
  for (int i = 0; i < roles.size(); i++)
    delete roles[i];
}

Output

Larry(1) Moe(2) Curly(3) Go(0): 2
Larry(1) Moe(2) Curly(3) Go(0): 1
Larry(1) Moe(2) Curly(3) Go(0): 3
Larry(1) Moe(2) Curly(3) Go(0): 0
Moe: slap head
Larry: poke eyes
Curly: suffer abuse


After

A clone() method has been added to the Stooge hierarchy. Each derived class implements that method by returning an instance of itself. A Factory class has been introduced that main- tains a suite of "breeder" objects (aka proto- types), and knows how to delegate to the correct prototype. 

class Stooge {
public:
   virtual Stooge* clone() = 0;
   virtual void slap_stick() = 0;
};

class Factory {
public:
   static Stooge* make_stooge( int choice );
private:
   static Stooge* s_prototypes[4];
};

int main() {
   vector roles;
   int             choice;

   while (true) {
      cout << "Larry(1) Moe(2) Curly(3) Go(0): ";
      cin >> choice;
      if (choice == 0)
         break;
      roles.push_back(
         Factory::make_stooge( choice ) );
   }

   for (int i=0; i < roles.size(); ++i)
      roles[i]->slap_stick();
   for (int i=0; i < roles.size(); ++i)
      delete roles[i];
}

class Larry : public Stooge {
public:
   Stooge*   clone() { return new Larry; }
   void slap_stick() {
      cout << "Larry: poke eyes\n"; }
};
class Moe : public Stooge {
public:
   Stooge*   clone() { return new Moe; }
   void slap_stick() {
      cout << "Moe: slap head\n"; }
};
class Curly : public Stooge {
public:
   Stooge*   clone() { return new Curly; }
   void slap_stick() {
      cout << "Curly: suffer abuse\n"; }
};

Stooge* Factory::s_prototypes[] = {
   0, new Larry, new Moe, new Curly
};
Stooge* Factory::make_stooge( int choice ) {
   return s_prototypes[choice]->clone();
}

Code examples

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