Hello again, lots of talking to each other today :) ----- Mensaje original ----- De: Robert Ramey <ramey@rrsd.com> Fecha: Sábado, Septiembre 16, 2006 8:03 pm Asunto: Re: [Boost-users] [serialization] Purpose of load_construct_data (was: Re: Library Interface Design)

JOAQUIN LOPEZ MU?Z wrote: [...]

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http://boost.org/libs/serialization/doc/ serialization.html#constructors where my_class serializes my_class::m *both* at my_class::serialize *and* my_class::(load|save)_construct_data. If the analysis I develop in my previous post is correct (is it?) this leads to my_class::m being serialized *twice* when a pointer to my_class is saved. Maybe it would sufice to rewrite this part of the documentation with a more sensible example like the one at test_non_default_ctor.cpp. What do you think?

I agree. The fact that the example uses a member variable confuses the issue. I'll amend the documentation accordingly.

Thank you!

However, the current example does illustrate a case where it seems impossible to avoid serializing a member twice. One could leave it in the load_construct_data, and eliminate it from the serialization code - but then this would work only if the object was serialized via a pointer. If it were my program I would say hmmmm - I'm really confusing the role of the variable m - If its to be initialzed at construction - it shouldn't be messed with by serialization or operator= etc. If its realy part of the objects dynamic state - then it shouldn't be required at construction time.

Well, I think the apparent puzzle stems from the incorrect assumption (IMHO) that a given member must be either part of the construction info or dynamic state --in many cases, it can be *both*, which is precisely what happens with my_class. I'd appreciate if you can consider this extension proposal that would allow for one-phase serialization while keeping the statu quo --maybe we can have our cake and eat it too :) Augment the serialization interface with a pair of new user-overridable function like this: template<typename Archive, class T> inline void construct_from_archive( Archive & ar, T * t, const unsigned int file_version) { // default implementation load_construct_data(ar,t,file_version); ar>>*t; } template<typename Archive, class T> inline void construct_to_archive( // OK, the name is ugly Archive & ar, const T * t, const unsigned int file_version) { // default implementation save_construct_data(ar,t,file_version); ar<<*t; } and use it when serializating T through a pointer. Now, if we return to the case of my_class, where my_class::m is both used at construction them and serialized as part of my_class::serialize, we can handle all situations as follows: // as in your example template<class Archive> inline void save_construct_data( Archive & ar, const my_class * t, const unsigned int) { ar << t->member; } // as in your example template<class Archive> inline void load_construct_data ( Archive & ar, my_class * t, const unsigned int) { int m; ar >> m; ::new(t)my_class(m); } // this is new template<class Archive> inline void construct_to_archive( Archive & ar, const my_class * t, const unsigned int fv) { save_construct_data(ar,t,fv); // avoid m being saved twice } // this is new template<class Archive> inline void construct_from_archive( Archive & ar, my_class * t, const unsigned int fv) { load_construct_data(ar,t,fv); // avoid m being loaded twice } My proposal basically amounts to adding a new customization point to the existing protocol for serialization of pointers. It has the following advantages: * You don't break anything (it's a pure extension). * It addresses a concern (one-phase serialization) that is conceptually different to the concerned addressed by (load|save)_construct_data (serializing non default- constructible objects), and does it so in an orthogonal manner. * It lets you solve the problem of members being both part of construction info and dynamic state, as I've (hopefully) just shown . How do you like this?

Anyway, I'll alter my example to make the variable in question "const" and remove it from serialization which is a better of example of the real cases where this can come up.

Robert Ramey

Best, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo