Automatic object factory in C++

Unlike other object oriented languages like java, C++ does not have much support for introspection. So for example it is not possible to request the list of subclasses of a given class.

In this post I am going to talk about a little trick that I found very useful when dealing with a lot of subclasses of a single base class. This is typically what we see in video game, where we have a base Object class that represents any kind of object in the game, and then one subclass for each type of object, like the player, enemies, etc.

In its simplest form, we can imagine a program that defines two kind of entities in the game, one being the player, and the other one an enemy. Both inherit from the same base class:

class Object
{
public:
    virtual void iter() {}
};


class Player : public Object
{
public:
    virtual void iter() {
        cout << "player iter" << endl;
    }
};

class Enemy : public Object
{
public:
    virtual void iter() {
        cout << "enemy iter" << endl;
    }
};

Typically we would then have a list of objects, and we can fill it with instances of either Player or Enemy. As the code grows, we can add other subclasses of Object.

Now the problem is that each time we add a new object in the game we need to call the proper constructor for the type we want, so if we want to add one player and two enemies the code might look like this:

objects.append(new Player());
objects.append(new Enemy());
objects.append(new Enemy());

However, sometime we don’t know in advance what type of object we want to add into the game. For example when we read a level file, we might decide the type by parsing some sort of id. So what we need is some sort of factory function that could create any type of object from an id. It might look like this:

// Static method of Object to create an object from a subclass name.
Object *Object::create(const string &name)
{
    if (name == "Player") return new Player();
    if (name == "Enemy") return new Enemy();
    ...
    return NULL;
}

Writing this function by hand is a bit tedious when the number of subclass become important. Fortunately we can do better than that:

First let’s define an abstract factory class, whose only virtual method returns a new instance of Object:

class ObjectFactory
{
public:
    virtual Object *create() = 0;
};

Then, we also add a static map of string to factory object to the Object class, and a static method to register a new factory into the map.

class Object
{
...
public:
    static void registerType(
        const string& name, ObjectFactory *factory)
    {
        fatories[name] = factory;
    }
private:
    static map<string, ObjectFactory*> factories;
};

So if we assume that the factories map is filled with items that map all the subclass names to a factory creating a new instance of the subclass, our global factory function can become:

Object *Object::create(const string &name)
{
    return factories[name]->create();
}

Now the only remaining problem is to fill the factories map automatically from the subclasses of Object. Turn out there is an elegant way to achieve that.

The idea is to add bellow each subclass a global static instance of a Factory subclass, whose constructor calls Object::registerType. Since global object constructors are called at startup before main, this will automatically fill our factory list. The code for the Player class would look like this:

// Player.cpp

class PlayerFactory : public ObjectFactory
{
public:
    PlayerFactory()
    {
        Object::registerType("Player", this);
    }
    virtual Object *create() {return new Player();}
};

static PlayerFactory global_PlayerFactory;

Since the code is always the same, we can turn it into a convenient macro:

#define REGISTER_TYPE(klass) \
    class klass##Factory : public ObjectFactory { \
    public: \
        klass##Factory() \
        { \
            Object::registerType(#klass, this); \
        } \
        virtual Object *create() { \
            return new klass(); \
        } \
    }; \
    static klass##Factory global_##klass##Factory;

And now all we have to do is add a line to register the types in each subclass cpp file:

// Player.cpp
REGISTER_TYPE(Player)

// Enemy.cpp
REGISTER_TYPE(Enemy)

And there is even an extra advantage: since we never have to access the subclasses directly -even their constructors- we can get ride of all the header files, and directly move the subclass definition into the cpp files to make the code smaller. #}