set a cmake variable if it is not changed by the user

Question

How can i (re)set cmake variable only when it is not changed by the user?

I have this variables:

set(DIR \"testdir\" CACHE PATH \"main directory\")
s         


        

Answer 1

Turning my comment into an answer

You could use MODIFIED cache variable property, but the documentation says

Do not set or get.

Maybe a better approach would be to check the modification with if statements:

set(DIR "testdir" CACHE PATH "main directory")
if (NOT DEFINED SUBDIR OR SUBDIR MATCHES "/subdir$")
    set(SUBDIR "${DIR}/subdir" CACHE PATH "subdirectory" FORCE)
endif()

Or you just don't put DIR inside SUBDIR and put the details into the description:

set(SUBDIR "subdir" CACHE PATH "subdirectory of main directory (see DIR)")