Re: [boost] [result_of] Allow result_of to work with C++11 lambdas

Subject: Re: [boost] [result_of] Allow result_of to work with C++11 lambdas
From: Eric Niebler (eniebler_at_[hidden])
Date: 2013-04-09 02:05:27

On 13-04-08 10:50 PM, Nathan Crookston wrote:
> Hi all,
> 
> Using C++11 lambda functions with several boost components.  For example,
> the following code snippet will not work with VC10, nor with g++ up to 4.8:
> 
> #include <boost/assign.hpp>
> #include <boost/range/adaptors.hpp>
> #include <boost/range/algorithm.hpp>
> #include <iostream>
> 
> int main()
> {
>   std::vector<int> v;
>   for(int i = 0; i < 10; ++i) v.push_back(i);
> 
>   boost::for_each(v | boost::adaptors::transformed([](int i) { return i *
> 2; }),
>     [](int i) { std::cout << i << std::endl; });
> }
> 
> g++4.5 gives around 34k of error messages, while VC10 has a mere 22k.
> 
> The root of these problems is that lambda functions don't conform to the
> tr1::result_of protocol -- there is no nested result_type member, nor
> nested result struct.  The standard doesn't mandate this case since
> decltype may be used to deduce the return type on C++11-conforming
> compilers.
> 
> boost::result_of disables the use of decltype on many compilers in favor of
> tr1-style deduction due to weaknesses in the implementation of decltype.
> While it is important to use tr1-protocol result_of to allow code involving
> incomplete types to compile, it would be very convenient to fall back to a
> decltype approach when it's detected that the tr1 protocol is not being
> followed.
> 
> Jeff Hellrung suggested[1] a fallback to decltype *only* for compilers
> which had nonconforming decltype operators.  Thus the only behavioral
> change would be that some code which before would produce an error would
> now compile and run correctly.
> 
> A ticket[1] with a patch (including tests and docs) was created.  Daniel
> Walker suggested further discussion on the list, which this attempts to
> provoke.  I think this would be a nice improvement for a set of often-used
> compilers -- the list of boost-users questions whose problems could be
> addressed by this patch is extensive (and can be provided, if desired).
> 
> Thanks,
> Nate
> 
> [1]
> http://boost.2283326.n4.nabble.com/Range-amp-c-0x-Lambdas-Can-this-be-done-td4638947i20.html
>

> [2] https://svn.boost.org/trac/boost/ticket/7753

For the record, I'm in favor of this. Daniel Walker is the maintainer of
result_of, but I've occasionally made changes myself. I haven't looked
at the patch in question, but it's been on my todo list for a long time.
My todo list has gotten pretty long, so if you or Daniel beat me to it,
bonus.

-- 
Eric Niebler
Boost.org