Re: [boost] range/iterators/algorithms return type deduction (was: Re: range_ex: status?)

From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2008-06-24 14:33:04

On Tue, Jun 24, 2008 at 1:19 PM, Giovanni Piero Deretta
<gpderetta_at_[hidden]> wrote:
> On Tue, Jun 24, 2008 at 7:07 PM, Daniel Walker
> <daniel.j.walker_at_[hidden]> wrote:
>> On Tue, Jun 24, 2008 at 12:51 PM, Daniel Walker
>> <daniel.j.walker_at_[hidden]> wrote:
>>> On Tue, Jun 24, 2008 at 12:12 PM, Mathias Gaunard
>>> <mathias.gaunard_at_[hidden]> wrote:
>>>> Steven Watanabe wrote:
>>>>> Can't you use result_of?
>>>>
>>>> As far as I can see, the current boost::result_of implementation doesn't
>>>> use decltype or typeof.
>>>
>>> The boost::result_of on svn trunk uses decltype when
>>> BOOST_HAS_DECLTYPE is defined. This happens automatically for gcc
>>> versions after 4.2.
>>
>> Correction - for gcc > 4.2, the boost::result_of on svn trunk uses
>> decltype only when you compile with -std=c++0x. It uses the old TR1
>> heuristic otherwise.
>>
>
> "__decltype()" works even outside of C++0x mode. Shouldn't it be used instead?

C++0x decltype and the TR1 heuristic are not completely compatible
(decltype always succeeds at return type deduction, the heuristic does
not), so I think it's good to only get the new and improved
boost::result_of when using the new and improved standard.

However, there is a need for improving result_of for compilers that
don't explicitly support C++0x decltype, so long as the user knows
when/how it's being improved. Boost.Typeof already aims to provide
aggressive type deduction for an array of compilers with varying
capabilities. As has been suggested, perhaps result_of could use
Boost.Typeof on C++03 compilers, which in turn could use various
compiler extension/hooks when available. So, for example, here's one
way of concisely providing several different strategies for return
type deduction based on the standard and/or compiler's capabilities.

- boost::tr1::result_of uses the legacy heuristic
- boost::cpp0x::result_of uses the standard decltype
- boost::cpp03::result_of uses Boost.Typeof
  (see the Boost.Typeof documentation for whether typeof, __decltype,
or the type registry is used)

For C++0x, boost::result_of would be an alias for
boost::cpp0x::result_of. For C++03, boost::result_of would be an alias
for boost::tr1::result_of, so as not to break any existing code. If
users want a better result_of for C++03 compilers, they can explicitly
upgrade from boost::result_of to boost::cpp03::result_of. And of
course, in practice, as compiler vendors supply C++0x standard
libraries, folks will simply use std::result_of. That's probably not
too much to keep track of as we migrate from one standard to the next.

Daniel Walker