Re: [boost] [serialization docs] Ping?

From: David Abrahams (dave_at_[hidden])
Date: 2005-09-22 08:04:48

Joaquin M Lopez Munoz <joaquin_at_[hidden]> writes:

> 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.

It's not intended to be a criticism, and no offense was intended.
Probably the quotation marks were misplaced, so, sorry for that.  I
didn't have time to evaluate what you wrote, so I really have no idea
whether it holds water.  That said, instinct tells me there's no way
to formulate this issue that really avoids the issue that no two
distinct objects in C++ are truly equivalent.

>> 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.

Correct, it is not defined.  You're expected to understand it because
it's a plain english word.

>> 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". 

Well, that would be one convenient definition, but since lots of types
aren't even syntactically EqualityComparable, that isn't much help.
"Assuming" it basically skirts the equivalence problem.

> 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.

Hey, sounds familiar!

> 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?

Exactly.

> 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.

I'm not sure it should be considered legit under any Archive concept
that will be defined by the library.  Is it a useful semantics?
Beware premature generalization!

> 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

Only if you think (3) is important.  And if you do, as I wrote
elsewhere, you can always make a weaker concept than Archive, that
allows (3).

> ultimately, archives are not responsible for holding the equivalence
> rule, 

It doesn't matter whether they're _ultimately_ responsible, if
Serializable also gives sensible guarantees.

> as they relay to user provided serialize() functions.

But that's not what Robert is saying; he's saying they don't have to
even do that!

> 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.

That's an interesting rule.  So essentially you are saying that the
output archive needs to record enough structure to ensure that the
input archive can read the same sequence of types?  

What if the user serializes an aggregate struct X containing two ints?
Is the corresponding input archive required to be able to read two
ints as part of reading an X?

>>  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.

I guess it depends whether you want something useful or just something
minimally restrictive.  Equivalence, even if not defined in the
standard, is still useful.  If we took out requirements such as

   If a==b and (a,b) is in the domain of == then *a is equivalent to *b.
   
from the input iterator requrements, and 

   t = u T& t is equivalent to u

from the assignable requirements, I assert that input iterators and
the algorithms would be much less useful.

That said, I find your approach interesting.  My instinct about the
need for equivalence here might be wrong, although I would still need
almost every Serializable type to provide an equivalence guarantee.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com