Re: [boost] Re: Re: [serialization] default-constructors

From: Dave Harris (brangdon_at_[hidden])
Date: 2003-10-06 04:17:58

In-Reply-To: <blqvc0$mhl$1_at_[hidden]>
ghost_at_[hidden] (Vladimir Prus) wrote (abridged):
> As Robert noted, you still have to make load_construct a friend of the
> serialized class.

No, you can instead overload it for the serialised class. You can add the 
overload without modifying the class. It is a non-intrusive approach.

I seem to say this in every message, yet people don't seem to get it. I 
don't know how to say it any clearer. I know you saw the example, because 
you comment on it below.

> Eh... you gave the following example:
> 
>  template<class Archive>
>    void load_construct(Archive & ar, my_class *pt, unsigned int version)
>    {
>        int a;
>        ar >> a;
>        new( pt ) my_class( a );
>        load( ar, *pt, version );
>    }
> 
> It's good, but it presumes that "save" method has written "a" for you to
> load. That's somewhat breaks duality between "save" and "load". Now 
> "save" should care about another function "load_construct", which is
> somewhat more complex.

In the current proposal, the pointer version of load:

    template<class Archive ar, class T>
    void load(Archive & ar,  T * &t,  unsigned int file_version);

is paired with a pointer version of save:

    template<class Archive ar, class T>
    void save(Archive & ar, const T * t, unsigned int file_version);

In the same way, in my suggestion load_construct needs to be paired with a 
save_construct (there may be a better name). The library uses 
save_construct to save in those situations where it knows it will be using 
load_construct to load. A user who overrides ones must also override the 
other.

Sorry I didn't mention this before - I thought it was obvious. Every load 
function needs a corresponding symmetric save function. Duality is 
preserved. Do you see any reason this can't be done?

> FWIW, in my use case it won't work, since I don't have any ctor which 
> can be used to reconstruct the class instance given field values. 

If there is no constructor, how do you make instances? You understand that 
your custom load_construct can follow construction with a normal 
serialisation load?

-- Dave Harris, Nottingham, UK