Re: C++11 move constructors

[Kjell Ahlstedt <kjell ahlstedt bredband net>]

2012-11-24 12:13, Chow Loong Jin skrev:
> On 24/11/2012 04:13, Murray Cumming wrote:
> > On Fri, 2012-11-23 at 14:50 -0500, Tristan Matthews wrote:
> > > 2012/11/23 Murray Cumming <murrayc murrayc com>:
> > > > Maybe we could add C++11 move constructors to the gtkmm classes, where appropriate, inside ifdefs to prevent them from breaking the build when C++11 is not enabled/supported by the compiler.
> > > How would you test for C++11? An AC_COMPILE_IFELSE test that builds a
> > > class with a move constructor?
> > Yes, I think that would be best.
> I think we should still have it disabled by default in the public headers, and
> opt-in using a macro that users can define manually.
>
> The reason for this is that move constructors will end up in public headers, and
> will break compilations in applications that are not compiled with -std=c++(0x|11).
Move constructors will probably end up both in public headers and in 
gtkmm's .cc files. If not all move constructors are inline or template, 
i.e. only in header files, a configure-time check is necessary. If gtkmm 
is configured and built without C++11 functionality (e.g. with gcc 
without -std=c++11), move constructors can't safely be enabled when an 
application program is compiled. The libgtkmm-3.0 library file will lack 
some necessary code. And you can't guarantee that a function is inlined. 
The 'inline' keyword is only a hint for the compiler, which it's not 
force to obey.

It would probably be possible to build gtkmm with C++11 functionality 
and then build an application without, if the move constructors in the 
header files are preceded by something like

#if GTKMM_CAN_USE_MOVE_CONSTRUCTORS && (__cplusplus >= 201103L || 
defined(__GXX_EXPERIMENTAL_CXX0X__))

where GTKMM_CAN_USE_MOVE_CONSTRUCTORS is defined in gtkmmconfig.h.

In the libsigc++ bug https://bugzilla.gnome.org/show_bug.cgi?id=672555 I 
have argued against using a configure-time check for checking if the 
compiler accepts the 'decltype' C++11 keyword, but that's another case. 
'decltype' is not needed when libsigc++ is built. It's only needed in 
some application programs.

Kjell