Share cmake variables between different folders - cmake

Question

Suppose I have a project setup like so:
Project
'-> CMakeLists.txt
'-> src
'-> CMakeLists.txt
'-> third_party
'-> CMakeLists.txt
In the CMakeLists.txt of third_party I have a variable set like so:
cmake_path( APPEND ${CMAKE_SOURCE_DIR} "third_party" OUTPUT_VARIABLE PROJECT_THIRD_PARTY_DIR )
In the top level CMakeLists.txt I have both 'third_party' and 'src' included and the third party folder has a particular prebuilt project as well as others that will be built. Is is possible to have access to the variable '${PROJECT_THIRD_PARTY_DIR}' from the src CMakeLists.txt or would I have to move the definition into the master CMakeLists.txt? From what I have found online it seems it can be done by using 'set( .... CACHE INTERNAL )' but I would prefer to use what I thought to be the correct (and newer) features of cmake 3.20 for this path variable.

Answer 1

First of all, you should minimize adding variables to your Cmake path.
Paths can change between operating systems and between different installations of the same operating system, so you should save information with Cmake variables.
Even if you are trying to use new Cmake, sticking with the set() command allows for better back-portability with earlier versions of Cmake. I would stay with the set(<blah name> CACHE INTERNAL ) call and move on.
Another note:
A variable defined in your top level Cmake file is accessible to the lower level CMake files. When the top level Cmake includes another Cmake file, the Cmake files included later also have access to the previous sub-Cmake file variables (with a few exceptions, like the filepath of the previous Cmake file). Ignoring the exceptions, the simple solution to me is to define your 3rd party sources first in your top level Cmake and then the later Cmake files in your own source code can access them.