[odb-users] How to define odb::section for a member defined in base class

[Boris Kolpackov]

Vladimir Yelnikov <vladimir.yelnikov at gmail.com> writes:

> I would like to declare a derived class demo2 with section tied with
> base member data but ODB compiler can't generate proper code with
> error pointing last pragma:
> 
> error: ')' expected at the end of db pragma section
>
> [...]
>
> #pragma db member(demo2::data) type("BLOB")  section(demo2::dataSection)

Quoting the manual (emphasis added):

"The section specifier indicates that a data member of a persistent class
belongs to an object section. The single required argument to this specifier
is the name of the section data member. This specifier can only be used on
*direct data members* of a persistent class."


> Is it possible to make things work?

You can use a virtual data member:

#include <deque>
#include <odb/section.hxx>

struct demo
{
  typedef std::deque<float> data_type;
  data_type data;

  int dummy;
};

struct demo2 : public demo
{

  int id;
  odb::section dataSection;
};

#pragma db object(demo) definition abstract
#pragma db member(demo::data) transient

#pragma db object(demo2)
#pragma db member(demo2::id) id
#pragma db member(demo2::dataSection) load(lazy)
#pragma db member(demo2::data_) virtual(demo::data_type) access(data) type("BLOB") section(dataSection)

Note that I've added a dummy non-transient data member to demo. If it
only needs to contain data, then you can make the whole demo class a
transient base (i.e., simply don't declare it as persistent).

Boris