Inheritance of gfc_symbol / gfc_component

[Janus Weil <janus at gcc dot gnu dot org>]

Dear gfortraners,

after GCC is converted to C++ now, I guess one might think about
taking advantage of this in the Fortran front end ...

One of the first things that comes to my mind, when thinking in this
direction, is that the structs 'gfc_component' and 'gfc_symbol' have a
lot in common (name, ts, attr, as, just to name a few), so that one
could think about having gfc_symbol inherit from gfc_component (in
Fortran this would be called "type extension", but I think the correct
C++ term is "inheritance"). There are a couple of routines in the
front end which basically act the same way on components and symbols
and which could be unified with such an approach (by receiving a
pointer, which can either point to a gfc_symbol or a gfc_component).
Examples: gfc_copy_formal_args, gfc_expr_replace_symbols,
resolve_procedure_interface.

Attached you find a very short and simple patch, which implements the
trivial part of this inheritance approach: It makes gfc_symbol inherit
from gfc_component, and removes those 7 members from gfc_symbol, which
are literally equivalent in both structs. (The patch compiles cleanly,
but is not regtested.)

In addition to these 7 'common' members, there are three more members
which are more-or-less equivalent, but named differently:
 * 'loc' vs 'declared at'
 * 'initializer' vs 'value'
 * 'next' vs 'components'
These could be replaced by purely mechanical efforts.

This leaves only two members of gfc_component, which are not shared by
gfc_symbol:
 * norestrict_decl
 * tb

In the current simple form of the patch, these would slightly blow up
gfc_symbol (being unused there). An alternative could be to set up a
base struct, from which both gfc_component and gfc_symbol would
inherit.

I would greatly appreciate some general comments on using C++isms in
the Fortran front end (and possibly further ideas in this direction),
and also on the inheritance idea proposed above. Does this make sense
to you? Further thoughts and ideas?

Cheers,
Janus

Attachment: gfc_symbol_inheritance.diff
Description: Binary data