Including CMake Project Libraries - c++

Question

I'm in the process of putting together a small c++ project using CMake for the first time. My current project structure is
├── bch
│ ├── CMakeLists.txt
│ ├── gf
│ │ ├── CMakeLists.txt
│ │ ├── include
│ │ │ └── gf.h
│ │ └── src
│ │ └── gf.cpp
│ ├── include
│ │ └── bch.h
│ └── src
│ └── bch.cpp
├── bsc
│ ├── CMakeLists.txt
│ ├── include
│ │ └── bsc.h
│ └── src
│ └── bsc.cpp
├── CMakeLists.txt
├── .gitignore
└── main.cpp
Currently I have gf as a subdirectory of bch. The contents of bch/CMakeLists is
cmake_minimum_required(VERSION 3.17)
project(bch VERSION 0.1.0)
# Targets
add_library(bch STATIC src/bch.cpp)
# Dependant
add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/gf)
target_link_libraries(bch PUBLIC gf)
# Export to dependants
target_include_directories(bch PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)
I would like to take the gf CMake project and place outside of the directory path of bch. This doesn't seem to be a supported structure when using the add_subdirectory command unless I'm missing something. Generally, what would be the current "Best Practice" for accomplishing my goal of decoupling the directory structure from the dependency tree?

Answer 1

If you want to decouple the project from dependencies, then I would suggest splitting cmake project into two separate, exporting the dependent target and then importing it with 'find_package'. Here is quick google find for that topic:
https://visualgdb.com/tutorials/linux/cmake/find_package
[edit]
For a more general approach I suggest cmake documentation:
https://cmake.org/cmake/help/latest/command/find_package.html#command:find_package
https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html
https://cmake.org/cmake/help/v3.18/command/install.html#export
The idea is:
In a dependency project to generate a '*ConfigVersion.cmake' file, install all needed files (headers, binaries and *ConfigVersion.cmake) using the 'EXPORT' in parameter the 'install' command.
In a final project use 'find_package' to import the dependency.
For bigger library projects I also suggest using namespaces to allow importing only selected parts of the library.