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Subject: [boost] [type_traits] Rewrite and dependency free version
From: John Maddock (boost.regex_at_[hidden])
Date: 2015-01-14 14:58:06

I'm going to throw this out there to see what folks think:

In a branch called "Version2", see is a partially
rewritten type_traits lib: it doesn't yet contain all the traits of the
old one, but there's a pretty good selection to test.

The main pros for this version are:

1) Zero dependencies other than Boost.Config.
2) No horrible macro-obfuscation - all "real code", making it easier to
read and maintain.
3) Substantially reduced template instantiations where practical - often
half as many as before.
4) Substantially reduced #includes - the aim is that all files should
include only what they need and no more - with what's needed determined
by the actual implementation variant when appropriate. There were also
a lot of spurious #includes left over from old workarounds no longer used.

The main cons:

1) I don't know what I've broken, but surely something, you can help by
testing but I would guess we would never be 100% sure it's all right
until it's out in the field. This is the main reason for avoiding this
until now.
2) MPL interoperability should still work, but files which expect
type_traits headers to have included mpl ones will be broken unless they
explicitly include what they need.
3) Current MPL inter-operability is fragile see
4) Function dispatching to an mpl::bool_ involves an (inline) function
call where as the old inheritance scheme did not. This should mostly
disappear once optimizations are turned on, but is still a potential
issue for some folks I suspect.
5) I haven't got to the "difficult" headers like common_type.hpp yet.
6) Whatever I've forgotten ;)

Of course (2), (3) and (4) could be solved by making mpl's bool.hpp some
kind of "core" header.

So... what I need from you kind folks is:

* Is this the right way forward?
* Can you test with your favourite compilers and compare with results
for develop - So far I've tested with msvc-10,11,12,14 Intel 14,15,
GCC-4.5.4...4.9.1, plus a reasonably recent clang.
* If anyone would like to help converting further traits all help would
be much appreciated.... Oh and the compiler requirements in the docs are
all wildly out of date too...

Regards, John.

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