Provide in the cmake configuration the C++ standard which was used to compile ROOT

[andriish]

Hi all,

It makes sense to compile software with ROOT using exactly the same C++ standard.
However, there is no nice way to obtain this information from ROOT cmake config.
Everything that is provided is a list of C++ flags.

Describe the solution you'd like

Have a CMake variable ROOT_CXX_STANDARD in ROOT cmake config.

Describe alternatives you've considered

Parsing the ROOT_CXX_FLAGS and hoping for the best.

Best regards,

Andrii

[amadio]

The better solution would be to improve ROOTConfig.cmake such that a given standard can be required when calling find_package, like find_package(ROOT REQUIRED cxx17). That way, the extra steps of checking the variable are not needed, and ROOT won't be considered found if it uses C++11, for example (in which case if you have multiple versions installed, CMake might actually search until it finds the right one).

[andriish]

Hi @amadio ,

that is an "active requirement". What I actually needed at some point is a "passive" information, an answer to a quaestion "What standard was used?". So these are similar, but yet different things.

Best regards,

Andrii

[andriish]

Hi @amadio ,

just found another code where people would benefit from this feature

https://github.com/acts-project/acts/blob/main/CMakeLists.txt line 104

Best regards,

Andrii

[eguiraud]

+1. Since ROOT requires that downstream projects be compiled with the same C++ standard that was used for ROOT, ROOT should expose that standard as ROOT_CXX_STANDARD.

Many projects have to extract that information by regex-matching, e.g. https://gitlab.cern.ch/hepmc/HepMC3/-/blob/master/CMakeLists.txt#L153-171 .

[amadio]

ROOT's targets already export the C++ standard they were built with, so if you just link your project with ROOT using those targets, you already get the same C++ standard used in your code. Worst case, you can call get_property on a target to check this. However, exporting this in a variable as well can make it easier for users indeed, as calling get_property is a bit cumbersome.

[eguiraud]

if you just link your project with ROOT using those targets

in reality most projects set their CMAKE_CXX_STANDARD which trumps the target's property.

you can call get_property on a target to check this

ah, that's already way better than regex-matching, thanks!

exporting this in a variable as well can make it easier for users indeed

👍

[dpiparo]

hi @bellenot : would it be hard to expose the standard in a potential new ROOT_CXX_STANDARD variable in your opinion?

[bellenot]

hi @bellenot : would it be hard to expose the standard in a potential new ROOT_CXX_STANDARD variable in your opinion?

No, but that requires quite some changes, also for the projects using ROOT. I'll investigate (I have plenty of changes for CMake on Windows anyway)

[amadio]

@bellenot If I understand correctly, what @dpiparo is suggesting is a simple

set(ROOT_CXX_STANDARD @CMAKE_CXX_STANDARD@)

somewhere in ROOTConfig.cmake.in.

[bellenot]

@amadio Oh! You're right, sorry, I misunderstood the requirement... Will make a PR now. Thanks!

[eguiraud]

Happy to see this moving forward!

also for the projects using ROOT

this should be sufficiently backwards-compatible. unless a downstream project was somehow setting and using a variable called like that already they should see no change.

(P.S. don't forget to update https://root.cern/manual/integrate_root_into_my_cmake_project )

[bellenot]

(P.S. don't forget to update https://root.cern/manual/integrate_root_into_my_cmake_project )

Right, thanks Enrico!

[bellenot]

@eguiraud Here is the PR for the doc: root-project/web#938

[github-actions]

Hi @bellenot,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖