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Subject: Re: [boost] Boost.Log 1.59.0 incompatible with MSVC option /Zc:wchar_t-
From: Giel van Schijndel (Giel.vanSchijndel_at_[hidden])
Date: 2015-10-28 13:03:23

On 28/10/15 17:25, Bo Persson wrote:
> On 2015-10-28 14:39, Giel van Schijndel wrote:
>> Hi,
>> I'm trying to build Boost.Log on MSVC9 (aka Visual Studio 2008) with the
>> [/Zc:wchar_t-][1] option (necessary due to a vendor's SDK we're using).
>> Unfortunately this fails to build.
>> The mentioned option turns wchar_t into a typedef to unsigned short
>> (i.e. it's not longer a distinct type).
>> TLDR:
>> My question _why_ is boost::log::aux::is_character_type<wchar_t>
>> excluded when BOOST_NO_INTRINSIC_WCHAR_T is #defined? I see no harm in
>> including it unconditionally.
>> For some reason however, the template meta-function
>> boost::log::aux::is_character_type<T>'s specialisation for wchar_t is
>> guarded by !defined(BOOST_NO_INTRINSIC_WCHAR_T) [2]. This means wchar_t
>> is _not_ regarded as a character type and prevents instantiation of the
>> boost::log::aux::code_convert functions [3] which are _still_ used.
>> The only reason I can see for excluding this is to prevent a _potential_
>> conflict with multiple specialisations for the same type (because of the
>> typedef nature of wchar_t). But the only other specialisation that could
>> potentially give a conflict is the one for char16_t, and that's mandated
>> by C++11 to be a distinct type (AFAIK the only reason why wchar_t isn't
>> always distinct type on Windows/MSVC is because they introduced the type
>> pre-standardisation, so I see no reason why that mistake should be
>> repeated for char16_t).
> What do you expect boost::log::aux::is_character_type<unsigned short>
> to return with this setting?

That would return 'true' because, with this option, 'wchar_t' and
'unsigned short' aren't distinguishable anymore. I don't think that's a
problem because is_character_type is only used as an internal
meta-function inside boost::log::aux::code_convert and those conversions
still make sense if 'wchar_t' is 'unsigned short'.

Met vriendelijke groet,
With kind regards,
Giel van Schijndel

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