How to set value by code to a CACHE variable defined by 3d party CMake?

Question

In my project, the CMakeLists includes other cmake files from a library and those dependencies need some cache variables to be configured by user values.

It is all working well if I define those values from the command line with the cmake command:
-DTHIRDPARTY_FRAMEWORK_ROOT="$thirdpartyFrameworkPath"

But can I define (= hardcode) such values in my own CMakeLists file?
To avoid my own users to do it when they configure my project (some values of the 3d party configuration are constant in my project), and make my own cmake interface simpler.

I tried to simply set the variable with a value, but it is both defined and used in the included cmake so it gets overwritten with their default value just before being used.
Using set(... FORCE) seems to work but it does not look clean to me, and might lead to confusing errors if they rename or change the type of the variables on their side. It also forces me to add a type and a doc string because of the set(... CACHE ...) syntax.

Is there a better way to do this?

Answer 1

Setting CACHE INTERNAL variable is a proper way for hardcode a parameter of the inner project in the outer one:

set(THIRDPARTY_FRAMEWORK_ROOT CACHE INTERNAL "Hardcoded root for 'thirdparty'" <value>)

INTERNAL type makes sure that this setting will overwrite the option (FORCE doesn't need) and makes sure that the option won't be shown for a "normal" user.

Since the parameter is not intended to be changed by a user, its real type is meaningless, so there is no needs for it to coincide with the one set in the inner project.

As for description, you could set it to be empty (the parameter is not shown to the normal user, remember?). Alternatively, in the description you could explain why do you set the variable in the outer project. So an "advanced" user will see your description.