Re: [boost] [clang] How to use CLang for Windows?

Subject: Re: [boost] [clang] How to use CLang for Windows?
From: Beman Dawes (bdawes_at_[hidden])
Date: 2013-09-29 20:37:01

On Sun, Sep 29, 2013 at 7:26 PM, Edward Diener <eldiener_at_[hidden]>wrote:

> On 9/29/2013 5:43 PM, Beman Dawes wrote:
>
>> On Sun, Sep 29, 2013 at 4:06 PM, Edward Diener <eldiener_at_[hidden]>*
>> *wrote:
>>
>>
>>> With all due respect, Beman, there is no point of doing this at least for
>>> Boost headers once I reverted the fix to the Boost PP code that would
>>> have
>>> allowed clang under Windows to correctly compiler Boost PP code as a
>>> strictly conforming C++ standard preprocessor. It fails miserably as the
>>> VC++ preprocessor and I am not personally interested particularly in
>>> discovering why since the VC++ preprocessor is just badly broken in a
>>> number of respects general. To emulate that brokeness cannot be the goal
>>> of
>>> any C++ compiler.
>>>
>>>
>> IIUC, they are aiming to support the aspects of VC++ that are required to
>> compile and run the libraries shipped with Visual Studio and libraries
>> like
>> Boost. I doubt they intend to mimic bugs not required for that purpose,
>> and
>> they have stated up front that they intend to support all C++11/14
>> features, not just those supported by VC++.
>>
>
> If they don't intend to "mimic bugs not required for that purpose" then
> the fix I put in Boost PP config.h is absolutely necessary. Please try
> "bjam toolset=clang" in the Boost PP test directory under Windows after
> building or installing clang under Windows. You will soon understand why I
> made the change I did. After the slew of failures that you see because
> clang, a highly comformant C++ preprocessor pretends to be VC++ for the
> purposes of the Boost PP code because it defines _MSC_VER, you can make the
> change I originally did and try "bjam toolset=clang" again in the Boost PP
> test directory. Then you will see the difference.
>
> I do understand and applaud clang's purpose to compile code properly under
> Windows, even using the Windows header files. But I do not see the purpose
> of emulating VC++ when it is broken in regards to the C++ standard if it is
> not necessary. My Boost PP change made emulation of the broken VC++
> preprocessor unnecessary for Boost PP. Please remember that very little
> other, if any at all, preprocessor code written for Windows is going to
> require clang to emulate the broken VC++ preprocessor in order to compile
> Windows code. Boost PP pushes the boundary of what can be done with the C++
> preprocessor in a cross-platform way. Can it really be that clang in
> Windows wants to go from a C++ standard compliant preprocessor to the
> rather horrible VC++ preprocessor just to accomplish its goal of compiling
> C++ under Windows ?
>
> Each case is distinct and there may be some other cases where clang will
> attempt to emulate VC++ but trying to duplicate the VC++ preprocessor
> should not be one of them. I cannot emphasize this more. There's no point
> in producing a first-rate product and then crippling it in some large
> respect because of emulation on a particular OS. I don't particularly care
> what else it brings to the table.
>
> I know it is a fine line to decide what compromises might have to be made
> in pure C++ standard code to emulate VC++ and therefore compile Windows
> code. There may be other cases where clang will emulate VC++ even if it
> does not strictly follow the C++ standard. But the preprocessor is not one
> of them if you know anything at all about how badly VC++'s preprocessor is
> broken for all but the most basic macro expansions.
>

OK, put your patch back in, and see if it helps.

Chandler has signed up for the developers list, so he can answer future
questions himself.

--Beman