CMake build target without specifying target_include_directories

Question

I have a project with the following file layout:

Project
├── CMakeLists.txt
├── app
│   ├── CMakeLists.txt
│   └── main.cpp
└── ext
    ├──CMakeLists.txt
    └── lib
        ├── CMakeLists.txt
        ├── include
        │   └── foo.h
        └── foo.cpp

foo is a 3rd party library that I downloaded the source code of that I want to use in main.cpp. I can build main.cpp using a cmake file like:

add_executable(app main.cpp)

target_include_directories(app PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/extern/foo/include)
target_link_libraries(app foo)

However I feel it should be the responsibility of the foo lib to specify what files should be included when using it. Is there a way to make this work without the target_include_directories call?

Answer 1

Just add

target_include_directories(foo PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)

into extern/foo/CMakeLists.txt.

That way the include directory will be accessible when you build foo library and for anyone who links with that library.

---

Note, that automatic propagation of include directory and other library properties works only when you link with the library target:

# Assuming you have 'add_library(foo)' somewhere,
# PUBLIC and INTERFACE properties of the `foo` library will be propagated
# to the 'app'.
target_link_libraries(app PUBLIC foo)

This would work even if add_library(foo) is issued after target_link_libraries call.

Propagation won't work when link with the library file:

# Propagation won't work with a name of the library file:
target_link_libraries(app PUBLIC foo.a)
# Propagation won't work with a full path to the library file:
target_link_libraries(app PUBLIC /path/to/foo.a)
# If 'add_library(foo)' in inaccessible from the project,
# then linking with 'foo' means linking with a library file,
# so propagation won't work.
target_link_directories(app PUBLIC /path/to/)
target_link_libraries(app PUBLIC foo)