David Abrahams <dave <at> boost-consulting.com> writes: [...]
As I've said before, you will need eventually to describe the relationship between compatible loading and saving archives. It will be something like
T x, y; // arbitrary operations on x to set its state sar & x; lar & y;
Postcondition: y is equivalent to x
[...]
Joaquin's post takes an "innovative" approach to the problem of specifying semantics but it isn't at all clear to me that it holds water.
I can do little to argument against that criticism. If you have specific concerns about the approach please do bring them here.
The reason that "equivalent" is a fuzzy term in C++ comes down to the fact that two distinct objects always have detectably distinct addresses, so no two distinct objects can _truly_ be equivalent.
As far I know, the only definitions for (object) equivalence in the standard are given in connection with strict weak orderings induced by comparison functors. Beside that, I failed to find any reference about what two objects being "equivalent" means.
Leaving aside that language corner, the idea of equivalence works perfectly well.
For the sake of the discussion, let's assume that "a and b are equivalent" is somehow defined as / related to "a==b". My thesis is that there are serious objections against this definition of equivalence in the context of serialization: 1. A serializable type need not be equality comparable. 2. "a==b" is a C++ expression, so implying that a and b are objects living inside the same program. If I save an object a on my PC, pass the file to you and you load it a year later as b on your Linux box, what is "a==b" supposed to mean? 3. A serializable type can be implemented without observing the "a==b" rule: for instance, a list-like container can load the elements in reverse order --I understand this is a perfectly legitimate implementation that shouldn't be banned because of the "a==b" restriction. One can argue that (1) and (2) can be overcome with a "fuzzier" definition of equivalence relying on the reader's intuition about this relationship, but (3), IMHO, breaks down any hope of attaching equivalence to serialization semantics: ultimately, archives are not responsible for holding the equivalence rule, as they relay to user provided serialize() functions. So, from my point of view, the real task of an input/output archive pair is to ensure that, when a T::serialize function is invoked on loading, the input context (i.e, permissible >> ops on the input archive) is a replica of the output sequence. This rule recursively descends to primitive (in the serialization sense) types, where an equivalence rule can actually be provided. My (skectchy) proposal is merely a formalization of this idea.
I suggest you use that, and the established conventions from the literature, to describe semantics. You have, essentially, an emergency on your hands -- this is not the time to try untested approaches. First plug the dyke and then, if you have time, think about a rewrite.
Without wanting to sound harsh, I think that what you propose as established conventions for describing serialization semantics hold little real information and, worse yet, can mislead readers to assume that Boost.Serialization is constrained by the equivalence rule when it is not (cf. point 3. above.) The current docs are better in this respect since at least they don't assert false semantic rules. Best regards, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo