[Openmcl-devel] CCL <-> C++ interface generator in alpha, reviewers wanted

dherring at tentpost.com dherring at tentpost.com
Fri Apr 8 08:53:09 PDT 2011

Jon Anthony wrote:
> Jason E. Aten wrote:
> ;; Since Common Lisp doesn't support method overloading (apparently,
> ;; please correct me if I've misunderstood the comments at the bottom of
> ;; page 194 of Practical Common Lisp),
> See the lambda list keywords: optional, rest and key.  For this simple
> example, optional looks like what you want.

In CLOS, generic functions distinguish between required and optional
keywords; dispatch only occurs on the required keywords.  (I feel I've
misstated the details, but the gist is ok.)  Conceptually, all CLOS
dispatch happens at runtime, like virtual functions in C++.  (CLOS
implementations may leverage compile-time constants and type information
to achieve static dispatch.)

C++ static dispatch follows a different ruleset; it can distinguish
between both the arity and type of arguments.  For virtual functions, C++
runtime dispatch invokes a function implementation stored in the vtable of
the first argument (left of the dot).

Here's one approach for expressing C++ semantics in CL.  (Warning: Code
typed over lunch and not tested.)

// C++
struct S
  virtual void f(int);
  virtual void f(float);
  virtual void f(int, float);

;; CL
(defstruct S ...)
(defgeneric f_1 (s x)
  (:method ((s S) (x int)) ...)
  (:method ((s S) (x float)) ...))
(defgeneric f_2 (s x y)
  (:method ((s S) (x int) (y float)) ...))
(declaim (inline f))
(defun f (&rest args)
  "arity dispatch for S::f in C++"
  (ecase (length args)
    (1 (funcall f_1 args))
    (2 (funcall f_2 args))))


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