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

Subject: Re: [Boost-users] [Range & c++0x Lambdas] Can this be done?
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2012-11-22 09:34:48

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.

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.

—John

Date: Wed, 21 Nov 2012 10:15:18 -0700 From: Nathan Crookston <nathan.crookston_at_[hidden]> 
To: boost-users_at_[hidden] Subject: Re: [Boost-users] [Range & c++0x Lambdas] Can 
this be done? Message-ID: 
<CAMb0s=1qtLcOiXTd5h7b2qkOSiYqjEVeN-m=rafJRHKDU6y6Cg_at_[hidden]> Content-Type: 
text/plain; charset="iso-8859-1" Rob, On Wed, Nov 21, 2012 at 6:39 AM, Robert Jones 
<robertgbjones_at_[hidden]>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
>