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

Context: my project consists of several executables and libraries. For a library, e.g. bar, I'd like to set a variable bar_INCLUDE_DIR which is added to the include paths of any depending executable.

There is a much better way to do this than to set variables in the parent scope. CMake allows a target to specify include directories, preprocessor symbols etc. that depending targets can use. In your case, you can use target_include_directories.

For example:

target_include_directories(my_target PUBLIC my_inc_dir)