Re: [Boost-users] [range] cannot use lambda predicate in adaptor with certain algorithms

Subject: Re: [Boost-users] [range] cannot use lambda predicate in adaptor with certain algorithms
From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2011-06-01 16:07:58

On Tue, May 31, 2011 at 9:37 PM, Michel MORIN <mimomorin_at_[hidden]> wrote:
> Daniel Walker wrote:
>> At some point, BOOST_RESULT_OF_USE_DECLTYPE may be defined by default,
>> probably when there's enough user interest, so thanks for cc'ing me.
>> The reason it is currently not defined by default is that it is
>> possible that it could introduce breaking changes to some C++03
>> function objects; e.g. if a function object defines result_type to be
>> some type other than the type of a call expression, then in C++03
>> result_of would give the user-specified result_type rather than the
>> actual result type, whereas in C++0x result_of always gives the actual
>> result type. For example:
>>
>> #include <boost/type_traits.hpp>
>> #include <boost/utility/result_of.hpp>
>>
>> struct f {
>>    typedef int result_type;
>>    double operator()(double);
>> };
>>
>> int main()
>> {
>>    using namespace boost;
>>    typedef result_of<f(double)>::type type;
>> #ifdef BOOST_RESULT_OF_USE_DECLTYPE
>>    static_assert(is_same<type, double>::value, "");
>> #else
>>    static_assert(is_same<type, int>::value, "");
>> #endif
>> }
>
> Thank you for explaining the problem.
> And yes, backward compatibility is always problematic.
>
> IMO, this use case is not the intended use of boost::result_of,
> so is it reasonable to deprecate this?

This is not an intended (or supported) use case at all. Nevertheless,
it seemed reasonable to roll out decltype slowly.

> And how about adding a "Note" that warns this kind of codes will break
> in the future to the documentation?

Sure. I added the following:

"In a future release, BOOST_RESULT_OF_USE_DECLTYPE may be enabled by
default on compilers that support decltype, so if you use the above
protocol please take care to ensure that the result_type and result<>
members are accurate. If you wish to continue to use the protocol on
compilers that support decltype, use boost::tr1_result_of, which is
also defined in <boost/utility/result_of.hpp>."

Thanks for the suggestions!

Daniel Walker