Versioned Records

Implied Version

• Just take sizeof(thing_t) as the version
• It's what Microsoft does for 20+ years. x.version=sizeof(thing_t) mantra is easy.

typedef struct _thing {
    int version;
} thing_t;

thing_t something;
something.version = sizeof(thing_t);
do_it_live(something);

Explicit Version

• Have a version field and always set it to the latest version (ex. a constant THING_CURRENT.)
• Full freedom to remove and replace old fields.
• Have to keep copies of old structures around forever though.

typedef struct _thing {
    int version;
    /* current version */
} current_thing_t;

typedef struct _thing {
    int version;
    /* fields when we changed some stuff */
} thing_v2_t;

typedef _thing {
    int version;
    /* fields from version one */
} thing_v1_t;

Entry points have to switch on the version then look up the runtime for the particular thing_v1_t, or, perform an upgrade procedure that brings that old version to current.