cmake: CMAKE_CXX_STANDARD overruled by Geant4_CXX_FLAGS in DDG4

[vvolkl]

When building DD4hep against a Geant4 installation that was built with a different CXX_STANDARD, the Geant4 one will be used to compile DDG4, as it becomes part of Geant4_CXX_FLAGS, which are in turn used to compile DDG4, at least in my build:

-- D++> [DD4hep] Geant4 CXX_FLAGS -W -Wall -pedantic -Wno-non-virtual-dtor -Wno-long-long -Wwrite-strings -Wpointer-arith -Woverloaded-virtual -Wno-variadic-macros 
-Wshadow -pipe -pthread -ftls-model=global-dynamic -std=c++14

This can lead to build errors when this is not compatible with the one used by ROOT.

root/spack/lib/spack/env/gcc/g++ -DBOOST_SPIRIT_USE_PHOENIX_V3 -DDD4HEP_USE_TINYXML -DDDG4_EXPORTS -DG4LIB_BUILD_DLL -DG4UI_USE_TCSH -I/tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDG4/include -I/tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDCore/include -I/tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDParsers/include -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/root-6.28.06-3ynbwb3imflq2qnrl3ophmt7o7knilmz/include/root -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/boost-1.83.0-zvpm7l67mphzg6bs733aqqysfnwyauzp/include -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/geant4-10.7.4-wrl76wz3d4xy5rxbtfow5yn6vswl2xyn/include/Geant4 -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/clhep-2.4.6.4-schzolggr2q3rt2pyz5wnsc6os6idxyu/include -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/zlib-ng-2.1.4-43pipt6aq3bztdhamxiy2jzxu4miaoox/include -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/expat-2.5.0-chqhbuyyswtm7knv46x3lozm4prsufie/include -isystem /root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/xerces-c-3.2.4-hwqxluk7hv42im7wp3ukpkyo6hrq67ev/include -ftls-model=global-dynamic -Wno-psabi -pedantic -Wextra -Wall -fdiagnostics-color=auto -Wdeprecated -Wno-long-long -Wformat-security -Wshadow            -pthread -O3 -DNDEBUG -std=c++17 -fPIC -W -Wall -pedantic -Wno-non-virtual-dtor -Wno-long-long -Wwrite-strings -Wpointer-arith -Woverloaded-virtual -Wno-variadic-macros -Wshadow -pipe -pthread -ftls-model=global-dynamic -std=c++14 -MD -MT DDG4/CMakeFiles/DDG4.dir/src/Geant4Converter.cpp.o -MF DDG4/CMakeFiles/DDG4.dir/src/Geant4Converter.cpp.o.d -o DDG4/CMakeFiles/DDG4.dir/src/Geant4Converter.cpp.o -c /tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDG4/src/Geant4Converter.cpp

...

/root/spack/opt/spack/linux-almalinux9-broadwell/gcc-11.3.1/root-6.28.06-3ynbwb3imflq2qnrl3ophmt7o7knilmz/include/root/TString.h:869:32: note: 'std::string_view' is only available from C++17 onwards
  869 |   std::string printValue(const std::string_view* val);
      |                                ^~~
/tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDG4/src/Geant4Converter.cpp: In member function 'void dd4hep::sim::Geant4Converter::handleProperties(dd4hep::Detector::Properties&) const':
/tmp/root/spack-stage/spack-stage-dd4hep-1.27-tpk4d52f757bndwiuyx2binippaeonbi/spack-src/DDG4/src/Geant4Converter.cpp:1222:20: warning: structured bindings only available with '-std=c++17' or '-std=gnu++17'
 1222 |   for( const auto& [nam, vals] : prp ) { 

I saw this as part of a spack build but should not be spack-specific in any way. It's not a bug per se, but I would have expected that the DD4hep CXX STANDARD is used throughout. Also, in my case, it's very possible to build DD4hep (cxxstd=17) against ROOT (cxxstd=17) and Geant4 (cxxstd=14) with some tweaks to the cmake config.

• OS version: alma9
• Compiler version: GCC 11.3.1
• Package version: 01-27
• Reproduced by: cmake .. ( using ROOT cxxstd=17 and Geant4 cxxstd=14 )
• Output:see text
• Goal: build DD4hep against ROOT cxxstd=17 and Geant4 cxxstd=14

[andresailer]

Fixed by #1191 in the sense that cmake now fails if the Geant4 standard differs from the DD4hep one.

[jmcarcell]

Aha I didn't remember this. In my case it was Geant4 with c++11 and then building with c++17 doesn't work because DD4hep needs at least c++14. To begin with, I'm not sure gcc should be happy when you pass --std=c++17 --std=c++11 and then keeps the second one, but it is 🤷