Re: [boost] [build] How to use with clang on windows?

Subject: Re: [boost] [build] How to use with clang on windows?
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-03-26 06:52:12

On 3/26/2016 12:58 AM, degski wrote:
> Hi,
>
>
> I don't really get what this means, could you please elaborate? What does
> "to use" and "the clang compatibility mode in the VC++ 14 IDE" mean. And
> what is "the preprocessor problem"?

The VC++ preprocessor is not a C++ standard conforming preprocessor. 
Both Boost PP and Boost VMD perform various tricks to get the VC++ 
preprocessor to mostly work with those heavily macro based Boost 
libraries. This working with VC++ in Boost PP and Boost VMD depends on 
identifying the VC++ compiler. Many other Boost libraries use Boost PP 
so using those other libraries with VC++ are affected by all the 
workarounds in Boost PP to get the non-standard VC++ preprocessor to be 
workable with Boost PP constructs.

The clang build which targets VC++ on Windows, as opposed to the clang 
build which targets mingw(-64)/gcc on Windows, 'emulates' the 
non-standard VC++ preprocessor so it can compile Windows API headers and 
VC++ header files. Exactly what VC++ preprocessor 'bugs' it emulates and 
how successful this emulation is I do not know, but I know it is done 
because I was told of it in the clang developer forum and I encountered 
a clang targeting VC++ preprocessor 'bug' which was explained to me as 
an emulation of VC++.

It was hard enough work for Paul Mensonides to get the VC++ preprocessor 
to work with Boost PP and it was hard enough work for me to get the VC++ 
preprocessor to work with Boost VMD that neither I nor, I assume, Paul 
wants to go back and figure out another set of preprocessor 'bugs' to 
get clang targeting VC++ to work with those libraries. For all I know 
clang targeting VC++ may indeed work with those libraries if the 
libraries pretend that clang targeting VC++ is VC++ for the intention of 
those libraries. Since I have been basically taking care of Boost PP and 
since I am the author of Boost VMD I have not tried clang targeting VC++ 
as VC++ in those libraries since I do not want to be tasked with fixing 
another set on non-standard C++ preprocessor bugs for another compiler 
if it does not work.

I made a suggestion to the clang developers that there 'emulation' of 
the non-standard VC++ preprocessor in their clang targeting VC++ 
implementation should be configurable so that when the compiler is not 
dealing with Windows API and VC++ header files it can be turned off and 
clang would then be the same C++ standard conforming preprocessor as it 
is with clang targeting mingw(-64)/gcc and clang on Linux and on the 
Mac. The suggestion was rejected. After that my interest in running 
clang targeting VC++ on Windows ended.

Unless you have dealt with it personally you cannot know how difficult 
it is doing the sort of preprocessor programming which Boost PP does and 
Boost VMD does with the VC++ preprocessor. Being asked to work with 
another preprocessor which 'emulates' the VC++ preprocessor in various 
unknown ways is not a task I wish on anybody.

I fully understand why the clang developers in their efforts to have 
clang on Windows work with VC++ had to 'emulate' the non-standard C++ 
preprocessor to work with Windows API and VC++ header files, since 
enough quirks in those header files have depended on the non-standard 
VC++ preprocessor. But propagating that non-standard preprocessor 
behavior to all preprocessor processing is IMO a major mistake, 
especially as the normal clang preprocessor is very good and C++ 
standard comnforming.