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Game data and serialization

Posted by EWClay, 05 April 2013 · 640 views

Getting data into a game is a big subject and I want to talk about some of the systems I'm using in some detail, so it may take a few posts to get through it. I will narrow the subject a little first though: this is not going to be about asset loading - textures, meshes, sounds and so on. Rather it's the data that makes up the game itself, so that would be levels, entities, and all the associated information. Since in the game these are C++ objects, there needs to be a mechanism for interacting with such objects in order to save and load them. So the first step is serialization, which is simply the process of translating an object into a stream of data and vice versa.

Obviously this is a well studied problem, but still a subtle one. C++ does not provide serialization out of the box. One of the best known solutions is the Boost Serialization library (http://www.boost.org/doc/libs/1_53_0/libs/serialization/doc/index.html). It's not entirely suitable for my purposes, for several reasons which I hope will become clear later, but it does offer some very useful concepts.

First among these is the separation of the process of serialization from the archive format. This is one of the fundamental goals of Boost Serialization; whether it achieves it is another matter. It also puts some care into the serialization interface so that it is fairly simple (at least, for simple classes) and economical in terms of the amount of code and maintenance required.

So, points that I like about Boost Serialization (and naturally intend to steal):
  • Templated serialization functions. Virtual base classes for archives is the alternative, but this seems to be the best implementation for flexibility and efficiency.
  • Non-intrusive code, as far as possible. Serialization has a minimal impact on the original class.
  • Automatic serialization of containers and other common types.
And what I don't like:
  • The versioning system. Versioning is explicit in Boost Serialization and I prefer it to be automatic, as in, I can add or remove members without having to write special code to do that.
  • Automatic object and pointer tracking. This complicates the archive format and the serialization interface. In contrast to versioning, I prefer this to be explicit.
  • Some aspects of the interface, including operator overloading.
Taking all that into account, here's a small example of a serialization function using my system:
struct Rect 
{
	int16 x, y;
	uint16 w, h;
};

template <typename Archive>
void Serialize(Archive& ar, Rect& t)
{
	ar.Serialize("X", t.x);
	ar.Serialize("Y", t.y);
	ar.Serialize("W", t.w);
	ar.Serialize("H", t.h);
}

As you can see it is templated to receive an archive and a serializable type. The function simply runs through the members and calls Serialize on the archive, providing a unique name (within this type) for each member. This function will do for loading and saving. The Serialize function on the archive is a template too and will detect the object type to serialize it correctly (and recursively).

And the output, when I write to a JSON archive:
{
	"Rect": {
		"X": 32,
		"Y": 0,
		"W": 32,
		"H": 32
	}
}
Now, that doesn't have to be a JSON archive. I'm using that for now because it's portable, easy to parse and human readable. But I could swich over to a binary format without changing the serialization function at all.

Next time I'll go through some more details of the implementation.




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