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

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

In-Reply-To: <01C38B87.6C788770_at_[hidden]>
ramey_at_[hidden] (Robert Ramey) wrote (abridged):
> The default implementation for deserializing a pointer is:
> 
> template<class Archive ar, class T>
> void load_construct(Archive & ar,  T * t,  unsigned int file_version)
> {
>       new( pt ) T(); // Default-construct in place.
>       ar >> *t;    // presumably
> }
>
> I don't see the improvement.  You're still stuck with giving
> load_construct access to a private constructor.

The improvement is that you can override load_construct so that it doesn't 
use the default constructor, and doesn't need friend. You are not stuck 
with this default implementation.

> >int f()
> >{
> >   Foo x[3];
> >   return x[0].value();
> >}
> 
> >The array in the code snippet above does not always exist.  It is
> >created when the function is entered and destroyed when it exits.
> 
> Hence need not be created/constructed by the serialization system which 
> is which is what is being discussed.
> 
> >Elements can be constructed at will in suitably aligned raw memory,
> >and an array will be created.
> 
> Serialization of the above created "array" requires no special 
> consideration.

Agreed. What I had in mind was more like:
    int f( Archive &ar ) {
        fixed_array<Foo,3> x( ar );
        return x[0].value();
    }

where fixed_array<T,size> is based on:
    unsigned char data[ size * sizeof(T) ]; // with alignment.

Getting this to work is non-trivial, though.

-- Dave Harris, Nottingham, UK