I can set the variable CMAKE_CXX_STANDARD like this
set(CMAKE_CXX_STANDARD 17)
but I can't print it like this
message("This is the C++ Standard we are using: ${CMAKE_CXX_STANDARD}")
I guess the variable CMAKE_CXX_STANDARD doesn't exist by default unless explicitly specified.
In such case, how is the default C++ Standard determined?
I guess the variable CMAKE_CXX_STANDARD doesn't exist by default unless explicitly specified.
Correct. If you need a specific C++ version you should specify.
In such case, how is the default C++ Standard determined?
If you don't specify it depends on the compiler. As an example new versions of MSVC for example default to C++14. As they don't want to maintain C++11 / C++98.
Not specifying the C++ standard is equivalent to not specifying other compiler flags. IE you are fine with the compiler provided defaults. Similar to how if you don't specify RTTI most C++ compilers will assume you want it enabled.
There are some projects that strive to work with whatever the compiler will give them, and a result won't specify the standard. This depends on what you want to support as a developer.
In most cases though it's a good idea to specify the C++ standard for your project.