wish: generation of type annotation for C++11 code.

[Basile Starynkevitch <basile at starynkevitch dot net>]

Hello All

(I am playing with C++11, but I am not a C++ expert, and I don't know the C++ front-end
part of GCC, so this is a feature wish only).

Perhaps it could be useful for some later (4.8?) release of GCC to produce an inferred
type annotation file.

For those knowing Ocaml, I was thinking of something similar to the -annot (or -dtypes)
option of the ocaml or ocamlopt compiler.
http://caml.inria.fr/pub/docs/manual-ocaml/manual022.html

For instance with the simplistic (single line) Ocaml file incr.ml:
let f x = x + 1;;
the incr.annot file obtained with ocamlc -annot incr.ml contains

#######
"incr.ml" 1 0 4 "incr.ml" 1 0 5
type(
  int -> int
)
ident(
  def f "incr.ml" 1 0 15 "_none_" 1 0 -1
)
"incr.ml" 1 0 6 "incr.ml" 1 0 7
type(
  int
)
ident(
  def x "incr.ml" 1 0 10 "incr.ml" 1 0 15
)
"incr.ml" 1 0 10 "incr.ml" 1 0 11
type(
  int
)
ident(
  int_ref x "incr.ml" 1 0 6 "incr.ml" 1 0 7
)
"incr.ml" 1 0 12 "incr.ml" 1 0 13
type(
  int -> int -> int
)
ident(
  ext_ref Pervasives.( + )
)
"incr.ml" 1 0 14 "incr.ml" 1 0 15
type(
  int
)
"incr.ml" 1 0 10 "incr.ml" 1 0 15
type(
  int
)
############

and external editors (in particular the ocaml or tuareg modes of Emacs) are able to parse
such annotation file, and interactively to show the type (with Ctrl-C Ctrl-T under
emacs) at the cursor's position.

 The main motivation would be to help C++ newbies, not C++ experts (those know well
enough the C++ language, and don't need help). With the type inference abilities given by
the auto keyword, it is sometimes hard for a beginner to understand what type is some
particular expression in his code (or what exactly function is called in an overloaded
context). The type annotation file can be generated by C++ front-end, and would contain
type and position information for every expression inside a source file.

That type annotation produced by g++ would be usable by external editors, etc.

Of course, such a thing could be done by some GCC plugin, but I believe providing the
feature inside GCC itself, and documenting the format of the textual type annotation
file, would give this feature more visibility, and it would help people a lot. And I am
pretty certain that the C++ frontend does compute the type of each expression, so it is
mostly the point of defining a textual type annotation format and output it.

The point is that inside languages with type inferences, like C++ is becoming in its 2011
standard, newbies are helped a lot with type annotations (and the editor's support of
them) - but experts rarely need this.

So I would dream of a -fannotate-type option to g++ 4.8 with a section in the
documentation describing the format of the type annotation file.

We already have -M (etc..) options to help about include dependencies.  We could have
some other option to help about type inferences.

Regards.
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
8, rue de la Faiencerie, 92340 Bourg La Reine, France
*** opinions {are only mine, sont seulement les miennes} ***