Modern CMake - setup libraries targets inside project - cmake

Question

My project structure presents as follows:
── src
├── app
│ ├── first_app
│ │ ├── include
│ │ └── src
│ │ └── CMakeLists.txt
│ ├── second_app
│ │ └── ...
├── lib
│ ├── first_lib
│ | ├── include
│ | └── src
│ | └── CMakeLists.txt
│ ├── second_lib
│ | ├── include
│ | └── src
│ | └── CMakeLists.txt
├── CMakeLists.txt
first_app uses both first_lib and second_lib. I would like to force CMake to find all dependencies (libraries and include headers) using
target_link_libraries(first_app first_lib::first_lib second_lib::second_lib).
I would like to ask two questions:
What command should be used to export first_lib::first_lib in ./src/lib/first_lib/CMakeLists.txt?
How to make first_lib::first_lib visible in ./src/app/first_app/CMakeLists.txt?

Answer 1

Supposing you create your lib using add_library(first_lib ...)
1) I would use:
include(GNUInstallDirs)
install(TARGETS first_lib
EXPORT FirstLibTargets
PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/first_lib
ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
)
src: https://github.com/Mizux/cmake-cpp/blob/master/Foo/CMakeLists.txt#L24
then you can install the export file using:
install(EXPORT FirstLibTargets
NAMESPACE first_lib::
DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/FirstLib
COMPONENT Devel)
src: https://github.com/Mizux/cmake-cpp/blob/master/CMakeLists.txt
note: I don't know if you can install target alias (i.e. install(TARGETS first_lib::first_lib) and/or have an export file with differents namespace (i.e. first_lib:: second_lib::)
2) simply use in first_lib/CMakeLists.txt:
add_library(first_lib::first_lib ALIAS first_lib) just after your add_library.
so first_app could always use first_lib::first-lib to retrieve this target dependencies what ever if you use a Meta CMake (i.e. add_subdirectory()) or find_package().
e.g. https://github.com/Mizux/cmake-cpp/blob/master/Foo/CMakeLists.txt