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From: Doug Gregor (dgregor_at_[hidden])
Date: 2006-11-09 12:01:11

My review of the function types library follows.

I've only been able to spend an hour or so reviewing this library,
but I've read through all of the documentation, browsed the code and
examples, and run the testsuite.

I've written and maintained several libraries that deal with function
types, and we end up reinventing the functionality in this library
over and over. We definitely need these facilities in Boost, if only
to have all of the dumb compiler workarounds in a single place. I
have a few comments, in no particular order:

        - This library should be merged into the type traits library, both
the code (headers go in boost/type_traits) and documentation (there
are already some function-type facilities there that should probably
be replaced). The facilities in this library are type traits, and the
library itself is small enough that it does not need to stand alone.

        - I'm not a huge fan of the tag types facilities. Part of the
problem with them is that there are so many new names (11), many of
which are very close to existing type traits: there's a tag
const_qualified, for instance, which basically means is_const on the
underlying function type. At the very least, these tags will need to
go in a sub-namespace of "boost" ("boost::type_traits" is fine). At
best, a crazy idea: is there a way to re-use existing type traits
like "is_const" instead of introducing these new names? Or, perhaps,
the "non_" versions could be mpl::not_, and "tag" could be
"mpl::and_". Interface reuse is really important!

        - I'm rather concerned about the portability of this library. Andy
Little has reported some failures on MSVC 7.1, and I'm seeing a huge
number of failures on Apple GCC 4.0.1. Function types will be a core
part of the type traits library: they absolutely must be portable.

        - If the library is accepted, I hope the author will help us clean
up other facilities in Boost (say, result_of <g>) to use this library
rather than their own hacks. Part of introducing infrastructure into
Boost is helping to make sure that infrastructure actually replaces
what it's meant to replace.

At this point, I don't know whether to accept or reject. The
portability problem can be solved, of course, and it will be solved
with time. The issue of tag types really bothers me, because the
interface is so large and seems so redundant with what we already
have in type traits. If tag types could be simplified, my vote would
be an enthusiastic "accept." Right now... I'm on the fence.


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