[cmake-developers] Language generator expressions

[Stephen Kelly]

Brad King wrote:

> On 05/16/2013 04:43 PM, Stephen Kelly wrote:
>> I've pushed the language-generator-expressions branch to my clone, and
>> the target-COMPILE_OPTIONS branch which depends on it, which should be
>> the implementation of
>> 
>>  http://public.kitware.com/Bug/view.php?id=6493
>> 
>> I hit a problem with the xcode and visual studio generators in that the
>> APIs of them do not provide a source file to use to determine the
>> compiler language at the point where I try to evaluate the compile flags,
>> defines and includes.
>> 
>> Is that a limitation of those IDEs? Or just of the generators? Can
>> someone familiar with those generators implement the porting to new APIs
>> I implemented in my branch?
> 
> The IDEs do not have per-language settings on a per-target basis.
> C and C++ are treated equally at that level and it is only at the
> per-source-file level that one can distinguish them.

Yes, that is the level I'm missing. I have two types of language genex - one 
for the COMPILE_LANGUAGE, and another for the LINK_LANGUAGE. 

The LINK_LANGUAGE is always available as target->GetLinkLanguage(config). In 
my patch to the makefile and ninja generators so far, the compile language 
comes from the source file being compiled, because the cmSourceFile happens 
to be available in the method where I need it already. I'm not familiar with 
those ide generators, but I'll assume for now that the cmSourceFile can be 
passed along to where I need it for the purpose of determining the compile 
language.


That way, given this:

 add_library(foo a.c b.cpp c.c d.cpp)
 target_compile_definitions(foo 
   PRIVATE
     $<$<COMPILE_LANGUAGE:C>:C_ONLY>
     $<$<COMPILE_LANGUAGE:CXX>:CXX_ONLY>
     $<$<LINK_LANGUAGE:CXX>:LINK_AS_CXX>
 )

a.c and c.c get compiled with -DC_ONLY, b.cpp and d.cpp get compiled with 
CXX_ONLY, and all get compiled with LINK_AS_CXX.

I have not added a way to determine if a language is enabled, and I don't 
know if such a thing would be useful.

Do you think this is the right direction otherwise?

Thanks,

Steve.