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From: David Abrahams (dave_at_[hidden])
Date: 2005-06-24 18:39:19

"JOAQUIN LOPEZ MU?Z" <joaquin_at_[hidden]> writes:

> De: David Abrahams <dave_at_[hidden]>

>> But all of this misses the high-level problem: the author of the code
>> doesn't know what he's doing. You simply can't serialize objects from
>> distinct scopes with tracking into the same archive, because there may
>> be aliasing.
> Totally agreed, this is what we are trying to detect
> in order to protect the author of the code.
>> And there's nothing we can reasonably do to detect that
>> problem when the aliased objects have the same type
> Nothing? I'm afraid I don't get you. A perfect
> aliasing detection mechanism is probably impossible
> to implement, but the hash test at least approximates it.
> This is better than providing no safety mechanism,
> as I understand you advocate.

I'm not sure it is. There's an imposition on users: all the types
they want to serialize have to support hashing.

It is nice that the serialization library automatically takes care of
hashing aggregated types and leaving out the unserialized data...

uh, wait: this will never work unless you plan only to do shallow
hashing. Otherwise you will get an exponential explosion for some
object graphs. Is that your intention?

> Or put another way: if the hash test fires the alarm,
> we are *sure* the user's code was incorrect. And
> if the user's code is correct, the hash test will
> pass.
> If you allow me to draw an analogy, this is similar
> to primality testing in maths: a 100% accurate
> primality test is complexity-wise unfeasible,
> but many statistical tests are fast, do not yield
> false negatives and get most of true negatives right.
> Am I being too cryptic? Sometimes my English skills
> play tricks on me.

No, the principle is familiar to me -- you don't need to try to
explain it. I'm getting more comfortable with the idea; it was hard
to accept at first because it looked too much like using const at
first, but in retrospect it is starting to look like a good idea if
the hashing is shallow and you can turn the checking off.

Dave Abrahams
Boost Consulting

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