Create objects based on a template of an existing object through cloning.
This is how it looks like:
Surprisingly enough, there are very few useful concrete examples of this, even in the literature. A lot of the time you see reference to ConcreteImplA and ConcreteImplB.
The original impetus for the Prototype pattern was actually:
- avoid subclasses of an object creator in the client application, like the abstract factory pattern does.
- avoid the inherent cost of creating a new object in the standard way (e.g., using the 'new' keyword) when it is prohibitively expensive for a given application.
That is actually quite interesting. As I mentioned in the Factory Method analysis post, I like the notion of using Factory Delegate (and thus avoiding subclassing) quite a lot. This is usually useful for behavioral objects, that contains little state (it would be more accurate to say that their state is behavior, such as a class that mostly contains delegate members for different things). But for those sort of things, you usually don’t really need to modify them after the fact, so there isn’t much of a prototype here.
The second reasoning is not relevant for most things today. The cost of new is so near zero to be effectively meaningless.
But something that isn’t mentioned about this pattern is that it is very useful for multi threading. The notion of being able to handle a cloned object that can be modified independently of its original is key in things like caches, as you can see in the code above. We make heavy use of that internally inside RavenDB, for example, although we choose a slight more complex (and performant) route.
A key observation about this is that Prototype assumes long lives objects. Because otherwise, there wouldn’t be the prototype instance to clone from. In wide variety of applications today, that is simply not the case. Most of our objects live only for a single request. And anything whose lifetime is longer than a single request is usually persisted to a stable storage, rendering the basis for the Prototype pattern existence moot.
Recommendation: This is still a useful pattern for a limited number of scenarios. In particular, the ability to hand out a copy of the instance from a cache means that we don’t have to worry about multi threading. That said, beyond this scenario, I haven’t found many other uses for this.