cmake variable scope, add_subdirectory

I have a CMakeLists.txt in my project root and one in my /src folder. The one in the /src folder only contains a variable with the .cpp files (set (SOURCEFILES main.cpp foo.cpp)) and in the root CMakeLists.txt I do add_subdirectory(src) and later I do add_executable(MyApp ${SOURCEFILES}).

But cmake gives me the error

add_executable called with incorrect number of arguments, no sources provided

How do I get cmake to see the variable? I read that cmake only knows global variables, but that's obviously not the case...

As mentioned in the documentation of the set command, each directory added with add_subdirectory or each function declared with function creates a new scope.

The new child scope inherits all variable definitions from its parent scope. Variable assignments in the new child scope with the set command will only be visible in the child scope unless the PARENT_SCOPE option is used.

To make the SOURCEFILES assignment visible in the root folder of your project, try:

set (SOURCEFILES main.cpp foo.cpp PARENT_SCOPE)