CMake override cached variable using command line - caching

Question

As I understand it, when you provide a variable via the command line with cmake (e.g. -DMy_Var=ON), that variable is stored inside the cache. When that variable is then accessed on future runs of the CMake script, it will always get the value stored inside the cache, ignoring any subsequent -DMy_Var=OFF parameters on the command line.
I understand that you can force the cache variable to be overwritten inside the CMakeLists.txt file using FORCE or by deleting the cache file, however I would like to know if there is a nice way for the -DMy_Var=XXX to be effective every time it is specified?
I have a suspicion that the answer is not to change these variables within a single build but rather have separate build sub-dirs for the different configs. Could someone clarify?

Answer 1

I found two methods for changing CMake variables.
The first one is suggested in the previous answer:
cmake -U My_Var -D Mu_Var=new_value
The second approach (I like it some more) is using CMake internal variables. In that case your variables will be still in the CMake cache, but they will be changed with each cmake invocation if they are specified with -D My_Var=.... The drawback is that these variables would not be seen from GUI or from the list of user's cache variables. I use the following approach for internal variables:
if (NOT DEFINED BUILD_NUMBER)
set(BUILD_NUMBER "unknown")
endif()
It allows me to set the BUILD_NUMBER from the command line (which is especially useful on the CI server):
cmake -D BUILD_NUMBER=4242 <source_dir>
With that approach if you don't specify your BUILD_NUMBER (but it was specified in previous invocations), it will use the cached value.

Answer 2

You could use
CMake -UMy_Var -DMy_Var=new_value
see documentation https://cmake.org/cmake/help/v3.9/manual/cmake.1.html
I hope this helps.

Answer 3

Find this post by coincidence.
It seems the behavior described by OP isn't the case for CMake 3.12 onwards. For previous releases, I didn't make some tests so cannot confirm.
Variables provided by -D on command line are stored in CMakeCache.txt. They can be overridden, even the same variable is repeatedly provided and the last one is set to the value for that variable.
For example, a very easy CMake script
message(STATUS "FOO = " ${FOO})
$ cmake -DFOO=123 -DFOO=321 .. # the last one takes effect
-- FOO = 321
-- Configuring done
-- Generating done
-- Build files have been written to: xxx
$ cmake .. # cache is remembered
-- FOO = 321
-- Configuring done
-- Generating done
-- Build files have been written to: xxx
$ cmake -DFOO=changed .. # override it
-- FOO = changed
-- Configuring done
-- Generating done
-- Build files have been written to: xxx