Re: [PATCH 1/2] Enable C++

[Petr Machata <pmachata@redhat.com>]

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Mark Wielaard <mjw@redhat.com> writes:

> On Fri, 2015-04-03 at 10:28 +0200, Petr Machata wrote:
>> - Detect C++ compiler in configure, add supporting infrastructure to
>>   eu.am.
>
> Looks good. Two questions below, but not blockers. Just curious.
>
>> index faf8add..f1d290b 100644
>> --- a/config/eu.am
>> +++ b/config/eu.am
>> [...]
>>  %.os: %.c %.o
>>  if AMDEP
>> +	test -d $(DEPDIR)/$(*D) || mkdir $(DEPDIR)/$(*D)
>>  	if $(COMPILE.os) -c -o $@ -fpic -DPIC -DSHARED -MT $@ -MD -MP \
>>  	  -MF "$(DEPDIR)/$*.Tpo" `test -f '$<' || echo '$(srcdir)/'`$<; \
>>  	then cat "$(DEPDIR)/$*.Tpo" >> "$(DEPDIR)/$*.Po"; \
>
> Why is that mkdir now necessary?

It's not, but I thought it's a good idea for symmetry.  The reason the
c++ bits need this mkdir is that we keep that code in a sub-directory,
and this creates the corresponding sub-directory for the dependency
tracking helper files.

>> +AC_CACHE_CHECK([for working C++ compiler], ac_cv_cxx_works,
>> +[AC_LANG_PUSH([C++])
>> + AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
>> +struct foo {
>> +  int i;
>> +  explicit foo () : i (0) {}
>> +  virtual ~foo () {}
>> +};])], ac_cv_cxx_works=yes, ac_cv_cxx_works=no)
>> + AC_LANG_POP()])
>> +AM_CONDITIONAL(HAVE_CXX, test "$ac_cv_cxx_works" = yes)
>
> We require gnu++98 above.
> Is the explicit enough to make sure the compiler supports C++98?

Well, this is a C++98 code.  It's of course not enough to test the whole
C++98--the language is vast.  The reason for that test is that I want to
make sure we really do have a C++ compiler.  AC_PROG_CXX happily exports
CXX even if none is available, so we can't use that in conditionals.

Thanks,
Petr