From: Lyle Johnson <ljohnson@...>
Date: 2002-02-05T22:40:25+09:00
Subject: Re: Ruby and Swig??

> are there examples in the RAA which are using swig??

FXRuby (http://fxruby.sourceforge.net) uses SWIG extensively to generate its
extension code. I suspect there are some other modules in the RAA using SWIG
but don't know for sure.

> I wonder when it is a good idea to use swig and when to
> write the extensions by hand ?
> are there performance differences ??

For small extensions (where the definition of "small" is up to you ;) using
SWIG is probably overkill. Ruby's API for writing extensions in C/C++ is so
easy to use that this is much easier than it would be for most other
scripting languages. But for larger libraries, and especially large C++
libraries, you might want to take a look at SWIG. The only disadvantage is
that the SWIG-generated source code tends to be larger than hand-written
code (although this is improving).

I don't think you'll see any measurable performance differences between
SWIG-generated extension code and hand-written extension code, because most
of the CPU time should be devoted to doing whatever "real work" it is that
the extension does. As an example, pretend you need to generate wrapper code
for some kind of scientific computing library written in C. The wrapped
version of one of these function calls will look something like this:

    VALUE wrap_TimeConsumingComputation(VALUE self, VALUE arg1, VALUE arg2)
{
        double x, y, result;
        x = NUM2DBL(arg1);
        y = NUM2DBL(arg2);
        result = TimeConsumingComputation(x, y);
        return rb_float_new(result);
    }

Now it is true that SWIG-generated code *may* be slightly more verbose than
this, perhaps introducing additional local variables to complete the
type-conversions; but the calls to NUM2DBL() and rb_float_new() will
presumably take *much* less time to complete than the call to
TimeConsumingComputation().

Hope this helps,

Lyle