Re: [Boost-users] [serialization] understanding of track_alwaysserialization trait

From: Robert Ramey (ramey_at_[hidden])
Date: 2007-12-24 17:21:45

"Yan Drugalya" <ydrugalya_at_[hidden]> wrote in message 
news:2441427d0712240501w2fca0f4em4bedfdeaf2981e3b_at_mail.gmail.com...
Folks ,

I feel a lack of understanding what actually track_always trait does. I 
expect that boost does not allow me to save/load twice object with the same 
address?

***
when tracking is enabled for a given type, the keeps  a collection of the 
addresses of all items saved and uses this to be sure that each item is only 
stored once.  So it will allow the same object to be saved more than once, 
but after the first time, only an object id is saved.

note that tracking is usally on/off by default so normally you don't have to 
specify it.

But it reports BOOST_STATIC_ASSERT even in following case:

****
This is a separate (though related issue which is described int eh 
"rationale" section of the documentation.

Question 1

Class A
{
// serialization support
};

BOOST_CLASS_TRACKING (A, track_always)
BOOST_CLASS_EXPORT( A)
 …
const A* a = new A; // at const here.
oa << a ;

I expect that it should raise above BOOST_STATIC_ASSERT because I trying to 
store the same object twice?

*** no - its raising the assert because the fact that the object is not a 
"const" suggests that you might be altering in the course of the 
serialization which would of course conflict with the concept of tracking 
which in turn suppress extra copies.

Question 3

Also I can't understand how track_always serialization trait works in case 
of Diamond inheritance

Quataion from documentation
"In a diamond hesitance structure with a virtual base class, object tracking 
will prevent redundant save/load invocations"
How boost knows that this is virtual base class and prevents redundant 
save/load invocations?

***
It doesn't understand the inhertance structure, it just suppresses redundant 
saves by keeping a record of what it has saved.

Please help me to find the way to understanding of object tracking trait :)

Thank you

You're welcome.

Robert Ramey

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