/ Design Patterns / Creational patterns / Factory Method

Factory Method 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. If we design an "extra level of indirection" (a "factory method") and have clients use it (instead of "new"), then the last bit of coupling goes away. The "factory method" (aka "virtual constructor") can be defined in the Stooge base class, or, in a separate "factory" class. Note that main() is no longer dependent on Stooge derived classes. 

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<Stooge*> 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 factory method is a static method of a class that returns an object of that class' type. But unlike a constructor, the actual object it returns might be an instance of a subclass. Another advantage of a factory method is that it can return existing instances multiple times. 

class Stooge
{
  public:
    // Factory Method
    static Stooge *make_stooge(int choice);
    virtual void slap_stick() = 0;
};

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

Stooge *Stooge::make_stooge(int choice)
{
  if (choice == 1)
    return new Larry;
  else if (choice == 2)
    return new Moe;
  else
    return new Curly;
}

Code examples

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