[sigc] Unwanted pass-by-value behavior with MSVC++ 7.1

["James Lin" <jameslin vmware com>]

Hi,

A couple weeks ago, I posted a problem with unwanted pass-by-value behavior I
ran into with libsigc++ 2.0.3 and MSVC++ 7.1. (I'm not sure if that message
ever got past the moderation system.)

Basically sigc::bind somehow was causing arguments to be passed by value even
when explicitly told to pass by reference. (I verified this by passing an
object by reference that had a private copy-constructor and getting a
compile-time error.  Also, test_lambda was producing incorrect results
(unrelated to the compiler optimization problem I mentioned earlier today).)

I'm not exactly sure why, but sun_forte_workaround appears to be the culprit.
(To get libsigc++ to compile at all with the MSVC++ 7.1 compiler, I need to
disable the LIBSIGC_TEMPLATE_PREFIX macro.  However, both
LIBSIGC_TEMPLATE_PREFIX and sun_forte_workaround are controlled by the
existence of SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD; disabling
LIBSIGC_TEMPLATE_PREFIX means enabling sun_forte_workaround.)

Anyhow, in adaptor_trait.h, changing:

  #ifdef SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD
    #define SIGC_WORKAROUND_OPERATOR_PARENTHESES operator()
  #else
    #define SIGC_WORKAROUND_OPERATOR_PARENTHESES sun_forte_workaround
  #endif

to:

  #if    defined(SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD) \
      || defined(WIN32)
    #define SIGC_WORKAROUND_OPERATOR_PARENTHESES operator()

  // ...

seems to fix my problem.  I guess the comment in adaptor_trait.h is correct:

  // TODO: This should have its own test, without using operator()
  #ifdef SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD
    #define LIBSIGC_TEMPLATE_PREFIX template
  ...

and that SIGC_WORKAROUND_OPERATOR_PARENTHESES and LIBSIGC_TEMPLATE_PREFIX
really do need their own, independent tests.

On a related note, all of the SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD
preprocessor conditionals test only for existence, not for value.  In
aclocal.m4, however, there's a line:

  AC_DEFINE([SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD],[0])

I'm not familiar with m4 or with autoconf, but I assume that this generates a
line:

  #define SIGC_TEMPLATE_SPECIALIZATION_OPERATOR_OVERLOAD 0

which would be wrong.  Shouldn't the preprocessor conditionals use #if FOO
instead of #ifdef FOO?

- James