Re: C++ pre-processor flags not updated by ax_cxx_compile_stdcxx.m4

[Moritz Klammler]

Are you using the latest version of `AX_CXX_COMPILE_STDCXX` [1]?  I
think this should have been fixed by Joshua's latest patch [2] which
changed the macro to append the `-std=c++14` flag to the `CXX` rather
than to the `CXXFLAGS` variable so the pre-processor should now pick it
up as well.

[1] https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
[2] 
http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commit;h=9e5ef8a7fa6f32dce37a9a340cef793254d2396b

I've tested with the following setup and it seems to work properly with
the latest version.

    $ ls
    configure.ac m4/

    $ cat configure.ac
    AC_PREREQ([2.69])
    AC_INIT([example], [1.0], address@hidden)
    AC_CONFIG_MACRO_DIR([m4])
    AC_PROG_CXX
    AX_CXX_COMPILE_STDCXX_14
    AC_LANG_PUSH([C++])
    AC_CHECK_HEADERS([initializer_list])
    AC_LANG_POP([C++])
    AC_OUTPUT

    $ ls m4/
    ax_cxx_compile_stdcxx.m4  ax_cxx_compile_stdcxx_14.m4

    $ autoreconf && ./configure
    # ...

With the older version of `ax_cxx_compile_stdcxx.m4` I get the following
bogus result.

    $ autoreconf && ./configure
    [...]
    checking initializer_list usability... yes
    checking initializer_list presence... no
    configure: WARNING: initializer_list: accepted by the compiler, rejected by 
the preprocessor!
    configure: WARNING: initializer_list: proceeding with the compiler's result
    checking for initializer_list... yes
    [...]



Peter Simons <address@hidden> writes:

> I'm re-sending the following message on behalf of Kip Warner
> <address@hidden> to the macro's authors:
>
> -----8<------------------------------------------------------------------------
> Hey list,
>
> Today while adding...
>
>     AX_CXX_COMPILE_STDCXX_14([noext],[mandatory])
>
>     ...
>
>     AC_CHECK_HEADERS([cfenv], [],
>     [AC_MSG_ERROR([missing some required standard C++ headers...])])
>
> ...to my configure.ac, I noticed a peculiar warning. The conftest.cpp
> successfully compiled without warning, but the pre-processor test
> raised one. Here is the log:
>
>     ...
>     configure:9708: checking cfenv usability
>     configure:9708: g++ -c -g3 -std=c++14 -mfpmath=sse -msse -msse2
>      conftest.cpp >&5
>     configure:9708: $? = 0
>     configure:9708: result: yes
>     configure:9708: checking cfenv presence
>     configure:9708: g++ -E  conftest.cpp
>     In file included from /usr/include/c++/5/cfenv:35:0,
>                      from conftest.cpp:31:
>     /usr/include/c++/5/bits/c++0x_warning.h:32:2: error: #error This
>     file requires compiler and library support for the ISO C++ 2011
>     standard. This support is currently experimental, and must be
>     enabled with the -std=c++11 or -std=gnu++11 compiler options.
>     ...
>
> I did a bit of digging and it looks like ax_cxx_compile_stdcxx.m4
> updates CXXFLAGS correctly, but doesn't touch CPPFLAGS. In most cases
> this probably wouldn't have mattered, but for this header it does.
>
> Should I simply add the following or do something else?
>
>     AX_CXX_COMPILE_STDCXX_14([noext],[mandatory])
>     CPPFLAGS="$CPPFLAGS -std=c++14"
>
> Or should ax_cxx_compile_stdcxx.m4 be patched to update the CPPFLAGS?
> -----8<------------------------------------------------------------------------
>
> Best regards,
> Peter

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