Variables set with PARENT_SCOPE are empty in the corresponding child scope. Why?

Question

Consider the following minimal example:

.
├── bar
│   └── CMakeLists.txt
└── CMakeLists.txt

where ./CMakeLists.txt is

Answer 1

Peter explained well the reason for this behaviour.

A workaround I usually use in this case is to set a cached variable, which will be visible everywhere:

set(BAR "Visible everywhere"
        CACHE INTERNAL ""
)

INTERNAL is to make it not visible from cmake-gui. INTERNAL implies FORCE, making sure it gets updated if you change something for example in your folder structure. The empty string is a description string, that you might want to fill if you believe it's necessary.

Note, though, that the correct approach is attaching properties to targets whenever possible, like using target_incude_directories, and propagate them to other targets by setting dependencies.