On Apr 9, 2013, at 2:05 AM, Eric Niebler
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
#include #include #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-t...http://boost.2283326.n4.nabble.com/Range-amp-c-0x-Lambdas-Can-this-be-done-t... [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.
I like the idea as well. I could devote some time to this and look at the patch again if there is any interest. - Daniel