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;
}