Question on type invariants

[reinkor <reinkor@gmail.com>]

This question may be a bit imprecise:

Assume a complex object A (for example a set of records stored in a Container or file). Let S (for example an array of records) represent a summary of A (derived 
from A).

I want to update S (call "Update_S") in an economical way. S may not be updated immediately each time A changes. And I would prefer not to update too much when S is up-to-date anyway (i.e. avoid redundant updating). I also want to keep things simple and avoid to combine updating S in any smart way when I change A (I may not have full control over the modifications of A anyway). The main thing is that S in "fresh"/updated when I use it (not necessary when A changes).

In principle I could mark A and S with a "last-modified-time-flag" so I can update S when I find it necessary. This has similarities to unix "make". Bad idea?

I can program the whole thing and keep the concepts in my head, but is it a natural "Ada way" to secure that S is up-to-date when used? Is "type invariants" of any help?

reinert