Re: [Boost-users] Library Interface Design

From: David Abrahams (dave_at_[hidden])
Date: 2006-09-13 11:34:02

"Thomas Matelich" <matelich_at_[hidden]> writes:

> On 9/13/06, David Abrahams <dave_at_[hidden]> wrote:
>> Jeff Garland <jeff_at_[hidden]> writes:
>>
>> > Loïc Joly wrote:
>> >> Jeff Garland a écrit :
>> >
>> >>> Oliver is correct -- serialization does not require default constructors for
>> >>> the types.  It does require a constructed object prior to reading in the data.
>> >>
>> >> Yes, you are right. My mistake.
>> >>
>> >> What I meant is that for deserialisation, you still have a two phases
>> >> construction: First, build your object with any mean available (if your
>> >> object have only constructors with non-default parameters, this will
>> >> probably imply building them with dummy parameters), then, in a second
>> >> phase, override the member values by the serialized version.
>> >
>> > Yep.
>>
>> Nope.  Sorry to be blunt, but I just want to make absolutely sure this
>> isn't missed:
>>
>> http://boost.org/libs/serialization/doc/serialization.html#constructors:
>>
>>   template<class Archive>
>>   inline void load_construct_data(
>>       Archive & ar, my_class * t, const unsigned int file_version
>>   ){
>>       // retrieve data from archive required to construct new instance
>>       int m;
>>       ar >> m;
>>       // invoke inplace constructor to initialize instance of my_class
>>       ::new(t)my_class(m);
>>   }
>>
>> One phase construction.
>>
>
> I poked around in the link you posted, but I don't see any examples of
> a my_class getting serialized into.  

That's exactly what's happening above.

> Where does t come from?  

It's memory allocated for you by the serialization library I suppose.

> IOW, how do I call this without passing dummy information into a
> my_class object?  

t isn't passed into the my_class; it's just raw memory with suitable
alignment.

> I'm sure I'm missing something, but all I can envision is a
> reinterpret_cast of a void*/malloc.  Or two one-phase constructions.

  new(t) X(m)

constructs a new X object in the memory at t, passing m as the one
ctor argument.

Does that help?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com