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Detect more c++ standards and make sure useful message actually appears #635

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merged 4 commits into from
Jul 8, 2024
Merged

Detect more c++ standards and make sure useful message actually appears #635

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e725f57
Detect more c++ standards and make sure useful message actually appears
tmadlener Jul 5, 2024
f9eb666
Order c++ standards numerically
tmadlener Jul 5, 2024
c8a0d77
Use a for loop to detect ROOT c++ standard
tmadlener Jul 5, 2024
e95aeb1
Add a bit more logging
tmadlener Jul 5, 2024
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6 changes: 5 additions & 1 deletion CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -93,8 +93,12 @@ if(NOT DEFINED ROOT_CXX_STANDARD)
set(ROOT_CXX_STANDARD 17)
elseif("cxx_std_20" IN_LIST ROOT_COMPILE_FEATURES)
set(ROOT_CXX_STANDARD 20)
elseif("cxx_std_14" IN_LIST ROOT_COMPILE_FEATURES)
set(ROOT_CXX_STANDARD 14)
elseif("cxx_std_11" IN_LIST ROOT_COMPILE_FEATURES)
set(ROOT_CXX_STANDARD 11)
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Is there a reason not to put these in order? 11, 14, 17, 20, else

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Mainly laziness. ;) I will order them

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@jmcarcell jmcarcell Jul 5, 2024 •
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Or simply a loop

foreach(std IN ITEMS 20 17 14 11)
  if("cxx_std_${std}" IN_LIST ROOT_COMPILE_FEATURES)
    set(ROOT_CXX_STANDARD ${std})
    break()
  endif()
endforeach()

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It's a loop now.

else()
message(FATAL_ERROR "ROOT C++ could not be detected")
message(WARNING "ROOT c++ standard could not be detected")
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This I don't understand, if the standard can't be detected then that's possibly going to cause some problems later on, no? And after 11 to 20 are included this shouldn't happen anyway? Didn't ROOT change not too long ago how they define the standard? That now would build fine but with maybe different standards in ROOT and podio.

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IIUC an undefined ROOT_CXX_STANDARD would be caught by this:

podio/CMakeLists.txt

Lines 101 to 103 in 76990a9

if(ROOT_CXX_STANDARD VERSION_LESS 17)
message(FATAL_ERROR "You are trying to build podio against a version of ROOT that has not been built with a sufficient c++ standard. podio requires c++17 or higher")
endif()

Which would at least tell people what a potential problem could be rather, than a rather cryptic error about not being able to detect a c++ standard.

The ROOT_CXX_STANDARD is only available from 6.32 onwards. So we still need this determination. From 6.30 onwards c++17 is required. I can't remember now when they switched the default c++ standard to 17, but given that I got an email today that someone ran into this issue with 6.28, I suppose not too long ago.

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Ah yes I was thinking cxx_std had been changed but it was ROOT_CXX_STANDARD: #540.

endif()
endif()

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