How to bump the C++ standard from the CMake command line?

user_225186 0
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Currently I have a project that needs C++17, therefore in the CMakeLists.txt I have this line pretty early on:

set(CMAKE_CXX_STANDARD 17)

From the command line (cmake) once in a while I want to test that the project also compiles with C++20. (to avoid surprises).

How can I choose to compile with C++20 from command line?

If I do cmake -DCMAKE_CXX_STANDARD=20 then it is later overwritten by the configuration instead of 17 being interpreted as just a minimum requirement.

I could check if the variable is predefined to avoid overwritting but I was looking for a more declative way to specify this.

(I am using cmake around 3.18.)

c++20 c++17 cmake c++
2 Answers

The solution is to remove that set command and use target properties instead:

# set(CMAKE_CXX_STANDARD 17)
target_compile_features(myexecutable PUBLIC cxx_std_17)

Then, setting -DCMAKE_CXX_STANDARD=20 on the terminal should work again.

user_21046970

Make CMAKE_CXX_STANDARD a cache variable. This allows you to easily specify a default that can be overwritten via the command line. Note that you need to make sure no "normal" CMAKE_CXX_STANDARD variable is available, since this value would take precedence over the cache variable.

set(CMAKE_CXX_STANDARD 17 CACHE STRING "the C++ standard to use for this project")

add_library(...)

Using cmake -D CMAKE_CXX_STANDARD=20 binary_dir should allow you to update the standard now, cmake -U CMAKE_CXX_STANDARD binary_dir should revert back to the default.

Alternatively you could create a custom cache variable. This would make a reuse easier, since it would allow you to overwrite the property, even if someone else sets CMAKE_CXX_STANDARD before using add_subdirectory to add your project.

set(MYPROJECT_CXX_STANDARD 17 CACHE STRING "the C++ standard to use for myproject")

# overwrite possibly preexisting value for this directory and subdirectories
set(CMAKE_CXX_STANDARD ${MYPROJECT_CXX_STANDARD})

add_library(...)
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