[Boost-users] [serialization] versioning for class nested within template class

Subject: [Boost-users] [serialization] versioning for class nested within template class
From: Gabriel Redner (gredner_at_[hidden])
Date: 2012-12-03 17:51:40

Hi folks,

I am having trouble incrementing the serialization version of a
regular class nested within a template class.  I'm not sure if the
problem is with boost or with my own understanding of the issue.

My code looks like:
==============================
template <typename A, typename B>
struct Foo
{
  struct Bar
  {
    int data1;
    int data2;

  private:
    friend class boost::serialization::access;
    template<class Archive>
    void serialize(Archive & ar, unsigned int version)
    {
      ar & data1;
      if (version >= 2) {
        ar & data2;
      } else {
        data2 = 0;
      }
    }
  };
};
==============================

My attempt at setting the version looks like:
==============================
namespace boost {
namespace serialization {

template <typename A, typename B>
struct version<typename Foo<A, B>::Bar>
{
  BOOST_STATIC_CONSTANT(unsigned int, value = 2);
};

} // namespace serialization
} // namespace boost
==============================

However, g++ gives an error:
==============================
main.cpp:31:8: error: template parameters not used in partial specialization:
main.cpp:31:8: error:         ‘A’
main.cpp:31:8: error:         ‘B’
==============================

Strangely, clang 3.0 compiles the code but issues some warnings:
==============================
main.cpp:31:8: warning: class template partial specialization contains
template parameters that can not be deduced;
      this partial specialization will never be used
struct version<typename Foo<A, B>::Bar>
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:30:20: note: non-deducible template parameter 'A'
template <typename A, typename B>
                   ^
main.cpp:30:32: note: non-deducible template parameter 'B'
template <typename A, typename B>
                               ^
1 warning generated.
==============================

I'd appreciate any pointers!

Thanks,
-Gabe