[petsc-dev] need c++ expert help

[Smith, Barry F.]

  There is always a C++ linker available so long as PETSc was built without --with-cxx=0 so just use it, why make a big deal out of it?

   Barry

> On Dec 31, 2017, at 5:58 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
> 
>>> On Dec 31, 2017, at 2:14 PM, Jed Brown <jed at jedbrown.org> wrote:
>>> 
>>> Matthew Knepley <knepley at gmail.com> writes:
>>> 
>>>> On Sun, Dec 31, 2017 at 2:35 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>> 
>>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>>> 
>>>>>> On Sun, Dec 31, 2017 at 1:55 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>>> 
>>>>>>> These look like linker errors and that build uses
>>>>>>> --with-cxxlib-autodetect=0.  We either need a rule to *link* C++ using
>>>>>>> CXX (i.e., mpicxx) or add LIBS=-lstdc++.
>>>>>>> 
>>>>>> 
>>>>>> I thought we were doing this (linking C++ mains with CXX).
>>>>> 
>>>>> After compiling to the object file (ex3.o) we don't know what language
>>>>> main was written in, and this doesn't solve the actual problem that C++
>>>>> linking is required if any object (not just main) depends on C++.
>>>>> 
>>>> 
>>>> My thinking here was the following:
>>>> 
>>>> 1) No C++ is allowed in PETSc, unless --with-clanguage=cxx, in which case
>>>> the linker is C++
>>>> 2) If C++ is in an external library, then that configure requires the C++
>>>> library
>>>> 3) The executable itself could be C++, in which case I proposed using the
>>>> C++ linker explicitly
>>>> 
>>>> About not knowing which objects come with C++ main: I thought we did. Don't
>>>> they go into a separate set?
>>> 
>>> Right, we do have that information for tests.  Note that a single test
>>> executable can depend on multiple source files, and the one containing
>>> main might not be C++ while another is.  I don't know if there are any
>>> such instances in PETSc.
>>> 
>>> I can change the build rules for tests with C++ sources later today.
>> 
>>   Jed, are you indicating that you will resolve the original problem I reported? Where will you resolve it? The problem may come up in one of my branches.
> 
> I was going to put it in a new branch from 'master'.  There is
> disgusting duplication in 'maint' that has since been fixed so I'd
> rather not do it there.
> 
> Under what conditions is PCC_LINKER different from CC (or CXX when
> clanguage=C++)?  I need to find a C++ linker and would prefer to use
> $(CXX) rather than adding a new configure test.  I feel like the
> generated petscvariables has an inordinate amount of duplication all
> with non-standard names and no documentation.
> 
> The cheap way to fix your issue is to add LIBS=-lstdc++ to your
> configure.