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

30 Sep 2013

      On Sun, Sep 29, 2013 at 7:26 PM, Edward Diener wrote:
...
On 9/29/2013 5:43 PM, Beman Dawes wrote:
...
On Sun, Sep 29, 2013 at 4:06 PM, Edward Diener *
*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