Re: [Boost-users] [serialization] Purpose of load_construct_data (was: Re: Library Interface Design)

I am not sure. I'm a little puzzled about the intended purpose of load_construct_data, as the docs and the code of Boost.Serialization seem to suggest that it is used for two incompatible tasks:

a) to construct a dummy object prior to deserialization itself, thus providing a user-overridable hook in case default construction is not appropriate. b) to deserialize in place as explained on the bit you quote.

The intended purpose was to address de-serialization of objects that have no default constructor. See test_non_default_ctor.

My hunch is that Robert started this with b) in mind but finally the facility was kept playing a more humble role as a). Consider for instance, serialization of pointers: currently, if an object instance is saved exclusively through pointers, Boost.Serialization does the following when the first such pointer is met

save_construct_data(ar,p); // S(0) ar<<*p; // S(1)

and correspondingly at loading time

p=get unitialized storage; load_construct_data(ar,p); // L(0) ar>>*p; // L(1)

By default save_construct_data does nothing and load_construct_data creates a default cted object, effectively leading to a two-phase construction scenario. Now, if the user decides to override (save|load)_construct_data for some class, say foo, so as to perform one-phase construction, we've got the following:

S(0) saves foo contents S(1) saves foo contents again

Nope. save_construct_data saves ONLY that data needed for construction while ar << *p save that data NOT used in construction.

L(0) loads foo contents and creates a foo object with these. L(1) loads foo contents again

Nope - see above. I believe this is well illustrated by the test_non_default_ctor.cpp example previously cited. Note that in generally, the data required to construct an instance might not be part of the instance itself. This is also addressed by this api. I'm not sure, but I suspect this could be also useful in de-serialization of objects which contain references to other objects as references can only be set at object construction time. Tracking would still work. Robert Ramey

leading to data duplication within the archive. This can be remedied by also overriding foo::serialize so as to do nothing, but then foo would be only serializable through pointers: serialization through instances

foo f(...) ... ar<

would do nothing!!

The only way in which load_construct_data would have served purpose b) is if it had designed so that

1: the default implementation for load_construct_data was: load_construct_data(ar,p){ ::new(p)T(); ar>>*p; } 2: the default implementation for save_construct_data was: save_construct_data(ar,p){ ar<<*p; } 3: Through-pointer serialization only consisted of steps S(0) and L(0) and omitted L(1) and S(1)

With this design, one would be able to override load_construct_data in the manner explained in serialization.html#constructors while retaining normal through-instance serializiation. As it currently stands, my view is that (save|load)_construct_data is not fit for one-phase deserialization.

Am I missing something? I hope Robert has the time to comment on this.

Joaquín M López Muñoz Telefónica, Investigación y Desarrollo