cmake, lost in the concept of global variables (and PARENT_SCOPE or add_subdirectory alternatives)

Question

I have a cmake project in which I have some modules and I\'m using Find-*.cmake for including the shared modules in the application. For not taking in account every module t

Answer 1

All variables in CMake are local by default. While you can use the PARENT_SCOPE parameter to increase the scope of a local variable by one layer, that mostly makes sense for return values of functions.

For find scripts on the other hand you usually want the behavior of a global variable: Once the find script is called by anyone, you want the results to be available everywhere. In particular, a second call to the same find script should just reuse the results of the first call. In CMake this is achieved by storing variables to the cache. The various find_* calls already do this automatically, so you should prefer using those where applicable. For any additional custom variables, set offers the capability to store to the cache as well:

set(MY_GLOBAL_VARIABLE "Some value" CACHE STRING "Description")

Note that local variables can hide cached variables of the same name in their scope.

Answer 2

You can 'simulate' GLOBAL variable behavior, by using properties with GLOBAL scope :

SET_PROPERTY(GLOBAL PROPERTY MyGlobalProperty "MyGlobalPropertyValue")

Then you can extract your global property by using

GET_PROPERTY(MyLocalVariable GLOBAL PROPERTY MyGlobalProperty)

Then, MyLocalVariable contains "MyGlobalPropertyValue".

Because PARENT_SCOPE extends variable definitions to the only parent directory (and not to its parents), there are cases it's not enough, for example if you have a deep source tree...