Mapping of C++ object arrays to C++ types is not supported

[karen-arutyunov]

Let's consider the following attempt to map a C++ object array to a C++ type using ODB project in-source build.

$ cat <<EOF >test.hxx
struct object_id
{
  object_id (int v = 0): value (v) {}

  int value;
};

typedef object_id object_ids[1];

#pragma db map type(object_ids) as(int) \\
  to((?)[0].value)                      \\
  from(conversion_function(?))

#pragma db value
struct object_refs
{
  object_ids refs;
};
EOF

$ <odb-src-root>/odb/odb/odb --database pgsql -I<odb-src-root>/libodb test.hxx

$ cat test-odb.hxx
  ...

  void access::composite_value_traits< ::object_refs, id_pgsql >::
  init (value_type& o,
        const image_type&  i,
        database* db)
  {
    ODB_POTENTIALLY_UNUSED (o);
    ODB_POTENTIALLY_UNUSED (i);
    ODB_POTENTIALLY_UNUSED (db);

    // refs
    //
    {
      ::object_id* v =
        o.refs;

      int vt;

      pgsql::value_traits<
          int,
          pgsql::id_integer >::set_value (
        vt,
        i.refs_value,
        i.refs_null);

      // From test.hxx:10:12
      v = conversion_function (vt);
    }
  }

  ...

Note that in the above generated function definition, a reference to the object_refs::refs member is (naturally) defined as ::object_id*. However, there is no such a possible from() clause expression which, being specified on the right hand side of the above v = ...; assignment, would modify the object_refs::refs member the v variable points to.

One of the possible solutions could be inventing a placeholder for the variable referring to the member. So that (given such a placeholder is (!)) we could write the from() expression as follows:

from((!)[0] = (?))

And so the assignment in the generated code would turn into:

      // From test.hxx:10:12
      v[0] = (vt);