Re: [boost] BOOST_NO_CXX11_HDR_CODECVT confusion

Subject: Re: [boost] BOOST_NO_CXX11_HDR_CODECVT confusion
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-11-17 14:02:46

On 11/17/2016 1:51 PM, Robert Ramey wrote:
> I use BOOST_NO_CXX11_HDR_CODECVT from config.hpp in order to selectively
> enable code which uses the built-in codecvt facets as opposed to the
> boost "home brew" one we've been using for a long time.  I do similar
> things with other macros such as BOOST_NO_CXX11_HDR_FORWARD_LIST.
>
> Problems arise when I use a recent compiler such as gcc 6.2 with the
> switch -std=c++03 as in
>
> "/hpc-6.2/usr/local/bin/g++"  -ftemplate-depth-128 -std=c++03 -O0
> -fno-inline -Wall -g -dynamic -gdwarf-2 -fexceptions -fPIC -arch x86_64
> -Wno-unused-variable -Wno-long-long -ftemplate-depth-255
> -DBOOST_ALL_NO_LIB=1 -DBOOST_FILESYSTEM_DYN_LINK=1
> -DBOOST_LIB_DIAGNOSTIC=1 -DBOOST_SERIALIZATION_DYN_LINK=1
> -DBOOST_SYSTEM_DYN_LINK=1 -DBOOST_WSERIALIZATION_DYN_LINK=1 -I"../../.."
> -c -o
> "../../../bin.v2/libs/serialization/test/test_iterators.test/darwin-6.2~03/debug/test_iterators.o"
> "test_iterators.cpp"
>
> In this ease I would expect to find that BOOST_NO_CXX11_HDR_CODECVT is
> defined as C++ standard does not include <codecvt> headers for versions
> prior to C++ 11.

Huh ? Where do you get that from ? Please see section 22.2.1.5 of the 
C++ standard 2003.

>  But the test matrix
> http://www.boost.org/development/tests/develop/developer/output/Sandia-gcc-6-2-0-boost-bin-v2-libs-config-test-config_info-test-gcc-6-2-0-debug.html
> for this compilers shows that BOOST_NO_CXX11_HDR_CODECVT is NOT defined
> when no -std=c++03 is used.
>
> OK - I see that for gcc 6.2 the default is C++ 14 if no switch is used.
> So the test matrix is OK.  I'm not sure what the column is meant to test
> as it does specify a particular version as the other columns do.  So I
> think the test for gcc 6.2 - and all the g++/clang compiler would be
> improved by specifying which version of the C++ standard they are
> actually testing.
>
> But I still have a problem.  When I use the swithc -std=c++03 I expect
> that BOOST_NO_CXX11_HDR_CODECVT be defined as C++ versions prior to C++
> 11 don't have header <codecvt>.  Same goes for other headers required
> only for more recent compilers. But my expectations aren't realized as
> BOOST_NO_CXX11_HDR_CODECVT is not defined in this case.  The leads to
> selection of library code which invokes an #error saying that it's not
> supported.
>
> Perhaps definition of BOOST_NO_CXX11_HDR_CODECVT is conditioned solely
> on the existence of <codecvt> rather than the existence of <codecvt> and
> the compiler running in C++ 11+ "mode".  I think this is what is messing
> things up for me.
>
> I might be able to work around this if I config.hpp included a macro
> which indicated which standard version it's intended to support.  It
> doesn't have this.  But I wouldn't need it if the BOOST_NO... worked as
> I would expect.
>
> Robert Ramey