[...rearranging...]
On Thu, Nov 22, 2012 at 6:34 AM, John M. Dlugosz <mpbecey7gu@snkmail.com> wrote:
Date: Wed, 21 Nov 2012 10:15:18 -0700 From: Nathan Crookston <nathan.crookston@gmail.com> To: boost-users@lists.boost.org Subject: Re: [Boost-users] [Range & c++0x Lambdas] Can this be done? Message-ID: <CAMb0s=1qtLcOiXTd5h7b2qkOSiYqjEVeN-m=rafJRHKDU6y6Cg@mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Rob, On Wed, Nov 21, 2012 at 6:39 AM, Robert Jones <robertgbjones@gmail.com>wrote:
Can the last line, labelled NOT Ok, be made to work? I think the lambda
does
not publish its result as a bind does, so I suspect it's hopeless. Any
thoughts?
// NOT Ok
boost::range::push_back( out, in | transformed( []( S & s ) { return
s.i; } ) );
}
It seems trivial to have a nested result_type in all cases with a lambda.
However, in C++11 there's no need for it due to decltype.
Sadly, some compilers fall into a gap where result_of (boost or std)
doesn't use decltype and lambdas don't publish tr1-style result_type --
leaving code like the above in the lurch.
Michel Morin wrote some code for adapting such a case which may be of
interest to you: http://lists.boost.org/boost-users/2012/01/72879.php
HTH,
Nate
I had the same issue and posted it a few months ago. I wrote to the MS
VS forum that their std::result_of should work correctly with both old
and new formats; in particular use the declared result_type if it exists
(which in general lets the user override the deduced type, great for
compatibility issues).
Actually doing that robustly is a bit of a maze of metaprogramming. I
ended up making a local my_namespace::result_of that I just handles what
I needed, and only when I asked for it. That doesn't help the existing
range templates, but such a change could be put back into
boost::result_of.
Maybe this is what you did, so I might not be suggesting something new, but... Perhaps boost::result_of could have an extra conditional logic branch added to it:
- If F::result_type exists, return F::result_type;
- Else:
- If BOOST_NO_CXX11_DECLTYPE, return F::result< F ( Args... ) >
- Else, if F::result<> exists, return F::result< F ( Args... ) >
- Else return decltype( declval<F>() ( declval< Args >()... ) )
Meanwhile, for different reasons, I made a macro that abstracts the
different syntax between C++11 lambdas and Apple Block Closures. The
latter generates a suitable lambda class complete with result_of member.
That makes me think that you could have a macro that wraps a
compiler-generated lambda expression in another class that inherits the
operator() etc. and adds in the result_of typedef to simply use
decltype.
Wouldn't that require defining a class at expression scope? Is that possible?
- Jeff