Create CMakeLists library with subdirectory sources - c++

Question

project
|
|----- src <-- Needs to be a library
| |
| |--- dir1 <-- Need not create a library
| | |--- dir1_1.cpp
| | |--- dir1_2.cpp
| | |--- CMakeLists.txt
| |
| |
| | --- dir2 <-- has an executable for testing and needs to be a library
| | |--- dir2.cpp
| | |--- dir2.h
| | |--- CMakeLists.txt
| |
| |
| | --- CMakeLists.txt
|
|
|
|----- CMakeLists.txt
How do I create CMakeLists in src such that it includes files from dir1 and dir2, such that only dir2 is a sub-project. I want to use add_subdirectory in src/CMakeLists.txt to add source files from dir1 and dir2. This way it will be easier to add more folders in the future.
I saw the use of PARENT_SCOPE but it also has some drawbacks. Is there a better solution? Maybe by using target_sources or something?

Answer 1

First of all, you should reconsider not making those subdirs self-contained modules, as modularity helps keep your projects cleaned.
Having said that, you can include in any variable defined in ./src/CMakeLists.txt any variable that's defined by ./src/dir1/CMakeLists.txt or ./src/dir2/CMakeLists.txt. Here's an example:
Here are the contents of ./src/dir1/CMakeLists.txt:
set(dir1_SOURCES
dir1/dir1_1.cpp
dir1/dir1_2.cpp
)
Here are the contents of ./src/CMakeLists.txt:
add_subdirectory(dir1)
set(src_SOURCES
${dir1_SOURCES}
)

Answer 2

You can create a library, then later add its source files in a subdirectory using target_sources. This gives you the flexibility to add more sources to MyLib or add more sub-projects later on, via additional calls to add_subdirectory.
It might look something like this:
src/CMakeLists.txt:
# Create a library
add_library(MyLib SHARED)
# Traverse to the 'dir1' subdirectory to populate the sources for MyLib.
add_subdirectory(dir1)
# Traverse to 'dir2' to create a sub-project.
add_subdirectory(dir2)
dir1/src/CMakeLists.txt:
target_sources(MyLib PUBLIC
dir1_1.cpp
dir1_2.cpp
)
# Add any headers from this directory.
target_include_directories(MyLib PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
dir2/src/CMakeLists.txt:
project(MySubProject)
add_library(MyLib2 SHARED
dir2.cpp
)
# Add any headers from this directory.
target_include_directories(MyLib2 PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
add_executable(MyExe ...)
...