[boost-users] [serialization] working example of (intrusive) serialization of class with no default constructor? and handling of nested classes with no default constructor

30 Sep 2011

      Hi,  

this question has been asked thousand of times and I still can not see a 
detailed answer.  How can I deserialize a class with no default constructor.
The usual response is to point to this documentation 
http://www.boost.org/doc/libs/1_35_0/libs/serialization/doc/serialization.ht... 
. I read the documentation many time and I can not figure out what does it 
mean and how it can be used.

Does any body has a working small example where this is effectively used?

Ideally, I am looking for something like the following, I am willing to 
accept other options, like making a2 a pointer, etc. 

int main(){
  {
     A a( ... ); // proper construction syntax
     std::ofstream ofs("A.xml"); 
     boost::archive::xml_oarchive oa(ofs);
     a & BOOST_SERIALIZATION_NVP(a);
  }
  {
     std::ifstream ifs("A.xml"); 
     boost::archive::xml_iarchive ia(ifs);
     A a2(ia);  // construct from iarchive or some other intermediate class
     {
         a2 & BOOST_SERIALIZATION_NVP(a2); //data can still be loaded in 
existing object (as usual)
     }
  }
  return 0;
}

Even if I want to implement my own reading of the xml_iarchive inside the 
constructor of A, it looks like I need to  access the nested elements of the 
class before accessing to the level of the class (I am thinking in xml terms 
here).

This is where I am stuck, the syntax above forces me to define

  A::A(boost::archive::xml_iarchive& ia){ ... }

If all members are default constructible I can do:

   A::A(boost::archive::xml_iarchive& ia){ 
     ia & BOOST_SERIALIZATION_NVP(*this);
   }
(yes, the solution in intrusive but the at least I avoid default constructor 
at all cost)

but if one of the member is not default constructible (and I can modify that 
class) I should be able to do something like, 

   A::A(boost::archive::xml_iarchive& ia) : member1( ...ia?... ){ 
      // ia & BOOST_SERIALIZATION_NVP(member2); // not possible because the 
archive is still waiting for class of type A to be read
   }

which has two problems, 1) I need to access the elements of the archive 
(e.g. member2 before A is really handled by boost.serialization) 2.a) for 
member1 it regresses the problem to the member1 type, which is fine because 
all this objects will at some point be composed only from build-in "default  
constructible" types 2.b) if I have no way to modify member1 and it is not 
default constructible then I can't continue but that is also fine because 
that means that member1 was not serializable anyway.

The real problem is 1). because if I have access to all the classes I will 
reach a point in which I have to read a built-in class from the iarchive, 
but I have have access to those nested elements directly, at least not with 
the documented interface.

Thanks,
Alfredo

alfC

Hazrat Pradipta Ranjali

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