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

Subject: [boost] [result_of] Allow result_of to work with C++11 lambdas
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2013-04-09 01:50:44

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