Object-Oriented 'Save Game' Techniques?

GreyArtist asks: "I took a course in C++ a year ago in which the instructor claimed that global (file-scope or inter-file-scope) variables were antiquated and not to be used under any circumstances. I immediately thought of a counter argument that involved the method I use for saving game data. The games (and many of the other programs) I write use not only global variables, but consecutive global variables declared in their own separate module. To save the game (or user settings) to file, I simply save a single large segment of data that contains all the necessary information. How do other coders do it? Would they create a 'MyObject.savemyself()' method for every object in their game? Do they save all the game code along with the data? Either way, it seems like a horrid case of code (or data) bloat. What do you die-hard object-oriented fanatics have to say about this, and what method they would you use for saving games?"

Comments

They're called singletons now

[mrami]

:) You make a singleton called "Prefs" and each module stores its values in a map under its own key (and in GCC, the key could be as easy as __PRETTY_FUNCTION__)

"Serialize"

[Ninja Programmer]

You simply have to model the the essential game state variables, then create a method for "serializing" them into something that can be thrown out to disk. There is no need at all for these variables to be global, just make sure you pass a "game context" down the call stack to any function which can modify the game state.

The reason why its important to have this abstraction, is that its required in order to make in-game demos, and to have any hope of writing a networked version of your game. It can also let you do strange things like split screen the game and let two people play independent games if you like (a speed contest, for example.)

How can this fail, let me count the ways...

[wowbagger]

Summary of your technique:
Declare various global variables.
Save game state into them.
On Save Game, write block of memory out.

How can this fail, let me count the ways:

1. Layout of global section can change from link to link due to project changes, thus making saves version dependant.
2. Non-save game global data can exist between save items, bloating save file.
3. You can get the size of the save block wrong, and end up not saving the data you need. e.g. you use &foo and &bar as your start and end pointers, but due to a change there are variables after &bar that need to be saved.
4. If the items you are saving become full-blown objects with compiler generated information (virtual function tables) you are overwriting that data with possibly incorrect data.
5. No error checking in file - so a recovered file may screw the game up.

And that's just what I can come up with before my morning coffee.

Look, I disagree with your instructer about "global variables are NEVER needed" - what, then are stdout/stderr/stdin/cout/cin/cerr, if not global variables?

However, global variables are like salt - a little may be needed, but too much will raise your blood pressure.

Again, this is before my morning coffee, but here's a couple of techniques that are better:

• Define a function to allocate a block of "saved space" - sort of a "save_malloc()" function. Allocate the objects you wish to save via that function. In that function, you grab a block of N bytes at initialization, you initialize an end pointer, and you "malloc" by moving the end pointer (no freeing allowed). You now know exactly what you need to save. You can also write a version # at the beginning of the file, and you can compute a checksum of the data. For OO types, you can fancy this up by writing a "Save_game" base class, and implementing new() for that base class.
• The more OO approach: Implement a "Save_game" base class. The base class implements a linked-list, with a static member as the head pointer. The class has Register/Deregister functions, and a pure virtual Size() method. Derived classes implement Size() { return sizeof(*this);} and call Register in their ctor. To save game, walk the list. This also allows you to save the size of the object (even better if you use RTTI, you can save the actual type of the object), and to checksum each object.

Singletons or Class variables.. all the same.

[mystran]

While I'd argue that global variables are usually a bad idea, I don't see a reason agaist file-scope variables. The rationale goes like this: In OOP you use Singletons when you only need a single variable of a given type. You do it, because while the Singleton gives you global access to the single variable, it still acts as an encapsulation method. This eliminates one of the problems of global variables: unpredicatable modifications, by allowing the Singleton to define what is allowed and what is not. Class variables can do the same, since only the method of the class can touch the variables (provided they are private). But file-scope (static) variables have the same property; the only thing different is that access is restricted not by a class, but by the compilation unit. You still get the same encapsulation. Another question is whether Singletons should be avoided too. Most of the time I like writing code in such a way, that you can run two (separated) instances of the program within the same process by simply create two objects of the main application class. Unfortunately Singletons are often needed to cope with libraries and APIs that assume there's only client within a process. IMHO Singletons (and class variables and methods) are to OOP what IO-monads are to functional programming: they are "hacks" that try to work around the limitations of a given programming model, and trying to minimize the damage caused.