On Tue, Jun 24, 2008 at 1:19 PM, Giovanni Piero Deretta <gpderetta@gmail.com> wrote:
On Tue, Jun 24, 2008 at 7:07 PM, Daniel Walker <daniel.j.walker@gmail.com> wrote:
On Tue, Jun 24, 2008 at 12:51 PM, Daniel Walker <daniel.j.walker@gmail.com> wrote:
On Tue, Jun 24, 2008 at 12:12 PM, Mathias Gaunard <mathias.gaunard@ens-lyon.org> 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