Re: [Boost-users] [Range & c++0x Lambdas] Can this be done?

[...rearranging...] On Thu, Nov 22, 2012 at 6:34 AM, John M. Dlugosz 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: Content-Type: text/plain; charset="iso-8859-1" Rob, On Wed, Nov 21, 2012 at 6:39 AM, Robert Jones wrote:

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Can the last line, labelled NOT Ok, be made to work? I think the lambda

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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.phphttp://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