How to detect if current scope has a parent in CMake?

Question

Is there any way to detect if the current scope has a parent?

I have a project that can either be a standalone project or a sub-project of another. To allow the sub project case, I use the PARENT_SCOPE flag to set() to push things up to the parent. However, when build as a standalone project I get a "current scope has no parent" warning. I would like to avoid that error by detecting if there is a parent and enclosing the set() calls in an if statement. Or is there another way to set a variable at parent scope only if there is a parent?

Answer 1

I think the most robust approach is to use the PARENT_DIRECTORY directory property.

This will yield the correct answer regardless of whether it's called before or after the project command, and regardless of whether the parent and child both have the same project name.

get_directory_property(hasParent PARENT_DIRECTORY) if(hasParent)   message(STATUS "Has a parent scope.") else()   message(STATUS "Doesn't have a parent scope.") endif() 

Answer 2

CMake version 3.21 added the PROJECT_IS_TOP_LEVEL global variable for this:

A boolean variable indicating whether the most recently called project() command in the current scope or above was in the top level CMakeLists.txt file.

project(my_project) [...]  if(PROJECT_IS_TOP_LEVEL)   message(STATUS "Is a top-level project.") endif()