Question

I’m attempting to implement a Save/Load feature into my small game. To accomplish this I have a central class that stores all the important variables of the game such as position, etc. I then save this class as binary data to a file. Then simply load it back for the loading function. This seems to work MOST of the time, but if I change certain things then try to do a save/load the program will crash with memory access violations. So, are classes guaranteed to have the same structure in memory on every run of the program or can the data be arranged at random like a struct?

Response to Jesus – I mean the data inside the class, so that if I save the class to disk, when I load it back, will everything fit nicely back.

Save

fout.write((char*) &game,sizeof Game);

Load

fin.read((char*) &game, sizeof Game);

Answer 1

Your approach is extremely fragile. With many restrictions, it can work. These restrictions are not worth subjecting your users (or yourself!) to in typical cases.

Some Restrictions:

• Never refer to external memory (e.g. a pointer or reference)
• Forbid ABI changes/differences. Common case: memory layout and natural alignment on 32 vs 64 will vary. The user will need a new ‘game’ for each ABI.
• Not endian compatible.
• Altering your type’s layouts will break your game. Changing your compiler options can do this.
• You’re basically limited to POD data.
• Use offsets instead of pointers to refer to internal data (This reference would be in contiguous memory).

Therefore, you can safely use this approach in extremely limited situations — that typically applies only to components of a system, rather than the entire state of the game.

Since this is tagged C++, “boost – Serialization” would be a good starting point. It’s well tested and abstracts many of the complexities for you.