Re: [sigc] C++0x lambdas

[Kjell Ahlstedt <kjell ahlstedt bredband net>]

From a post to libsigc-list 2012-01-06

> That said, C++98 code could also benefit from this addition since it allows the
> use of out-of-sigc++ functors. I have managed to amend the implementation to
> compile with -std=c++98, but it requires BOOST_TYPEOF. Perhaps we could isolate
> the BOOST_TYPEOF implementation for inclusion into sigc++?
>
> Kind regards,
> Loong Jin


Hi Loong Jin and others,

This is a late continuation of a discussion on libsigc-list in January 
2012 (it started in August 2011).

Some time ago I filed bug 
https://bugzilla.gnome.org/show_bug.cgi?id=672555, suggesting that 
libsigc's lambda functions shall be deprecated, when they can be 
replaced by C++11's lambda functions.
It's probably too early to deprecate them now, but I want to make it 
easier to use C++11 lambda functions.

The specialization of sigc::functor_trait that you presented in
https://mail.gnome.org/archives/libsigc-list/2012-January/msg00000.html
is interesting.

I'm also interested in the amendment you mention that compiles with 
-std=c++98. Are you willing to show it? It's not very attractive to 
include BOOST_TYPEOF in libsigc++, but still I'm interested, to see if 
it's something that I should consider for my bug report.

I managed to simplify your code a bit:

//-----------------------------------
#include <sigc++/functors/functor_trait.h>

namespace sigc
{
  template <typename Functor>
  struct functor_trait<Functor, false>
  {
    typedef typename functor_trait<decltype(&Functor::operator()), 
false>::result_type result_type;
    typedef Functor functor_type;
  };
}
//-----------------------------------

It still doesn't work if the functor contains an overloaded operator(). 
It can't. Then "&Functor::operator()" is ambiguous.
I've found another solution for that case. It's only slightly less easy 
to use, compiles with -std=c++98, and requires only a moderate amount of 
new code in libsigc++. Used like so:

class MyFunctor { ..... };
sigc::slot<bool, int> slot1 = sigc::oper_fun<bool>(MyFunctor()); // One 
or more operator(), C++98 or C++11

Regards
Kjell