Re: conditionally compiling C++ code

[Sander Niemeijer]

I was just wondering, but aren't the --with-PACKAGE/--enable-FEATURE 
options of configure meant for this?

I have a package that can optionally make use of the HDF4 library 
(which, if included, provides extra functionality to import/export data 
in HDF4 format). I dealt with this in my configure.ac as follows:

---
AC_ARG_WITH([hdf4],
  [AC_HELP_STRING([--with-hdf4],[build HDF4 support into this 
package])],
  [ac_cv_with_hdf4=$withval],
  [AC_CACHE_CHECK([use HDF4],ac_cv_with_hdf4,ac_cv_with_hdf4=no)])

if test $ac_cv_with_hdf4 = yes ; then
  ... <Check for HDF4 libs and headers via environment variables that 
specify their location and set HAVE_HDF4 definition and 
ac_cv_have_hdf4> ...
  if test $ac_cv_have_hdf4 = no ; then
    AC_MSG_ERR([HDF4 libraries and/or header files are not found. Try 
setting the appropriate environment variables.])
  fi
fi
---
This approach forces you to explicitly say that you want to make use of 
the HDF4 functionality and if so it will give an error if it could not 
find the HDF4 libs/headers.

In my view there should only be a 'required' and 'not needed' status 
for libraries. And if some functionality is optional (which may require 
one or even more libraries) this should be handled through configure 
options.

Just my 2 cents.

Regards,
Sander

On dinsdag, apr 29, 2003, at 12:54 Europe/Amsterdam, Patrick Welche 
wrote:

> On Tue, Apr 29, 2003 at 06:12:15AM -0400, Thomas E. Dickey wrote:
> > On Mon, 28 Apr 2003, Bob Proulx wrote:
> >
> > > Thomas Dickey wrote:
> > > > Peter Eisentraut wrote:
> > ...nonsense
> > > As I read Peter's post I believe the point was that the tool was
> > > generally considered to require readline functionality.  Not having 
> > > it
> >
> > readline is a library (if the configure script fails, readline 
> > wouldn't
> > be linked except if the configure script is doubly-incorrect).
> >
> > The supposedly analogous example of an external program is implausible
> > as well (it's common practice to distinguish between nice "optional"
> > stuff versus essentials - configure is properly used for the latter).
>
> It seems that we are trying to differentiate between libraries which 
> are
> - required     (error: won't work without it)
> - recommended  (warning: some usual/useful features won't work)
> - optional     (just don't link with it, user probably won't see the 
> difference)
>
> How about another variable to AC_CHECK_LIB, which
> if "x$6" = "xREQUIRED" && test $ac_somelib = none; then
>   AC_MSG_ERROR...
> where variable is REQUIRED, RECOMMENDED, (OPTIONAL | "")
>
> ?
>
> Cheers,
>
> Patrick