Re: [Boost-users] Library Interface Design

13 Sep 2006

      "Thomas Matelich"  writes:
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On 9/13/06, David Abrahams  wrote:
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Jeff Garland  writes:
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Loïc Joly wrote:
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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.
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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