CMake - dependencies (headers) between apps/libraries in same project - c++

Question

I have the following project structure:
CMakeLists.txt
lib1/CMakeLists.txt and all cpp and header files of the lib
lib2/CMakeLists.txt and all cpp and header files of the lib
app/CMakeLists.txt and all cpp and header files of the app
The main CMakeLists.txt looks like:
PROJECT( ${PROJECT_NAME} )
add_subdirectory(lib1)
add_subdirectory(lib2)
add_subdirectory(app)
The lib1/CMakeLists.txt looks eg like (stripped):
SET(SOURCE
file.cpp
)
SET(HEADERS
some_lib_header.h
)
add_library( lib1 ${SOURCE} ${HEADERS} )
and the one for the app looks the same except of ADD_EXECUTABLE:
SET(SOURCE
main.cpp
)
SET(HEADERS
some_header.h
)
add_library( lib1 ${SOURCE} ${HEADERS} )
ADD_EXECUTABLE( app ${SOURCE} ${HEADERS} )
I found the setup working well this way because out of this, I can generate one Visual Studio solution file which contains all those three projects. But my problem is that my app includes header files of lib1 (and also of lib2, which depends on lib1). When I do
$mkdir build
$cd build
$cmake -C ..\myproject
it generates out-of-source VS .sln file as I want it, but the app doesn't compile because it can't find the header files of lib1 (obviously).
Now I read and tried many things, like TARGET_LINK_LIBRARIES( app lib1 ) (which got the app to link with the lib1, but not solve the header include issue), and things like add_subdirectory( ../lib1 ) in various variants in the CMakeLists.txt of app (which all throwed errors that I couldn't fix), and also find_package (which I guess is the wrong approach).
So how can I solve this (I guess simple...) problem?

Answer 1

Here's one possible solution:
Root CMakeLists.txt:
cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
project(${PROJECT_NAME})
add_subdirectory(lib1)
add_subdirectory(lib2)
add_subdirectory(app)
lib1/CMakeLists.txt:
project(Lib1)
add_library(lib1 lib1.cpp lib1.h)
lib2/CMakeLists.txt:
project(Lib2)
add_library(lib2 lib2.cpp lib2.h)
# Add /lib1 to #include search path
include_directories(${Lib1_SOURCE_DIR})
# Specify lib2's dependency on lib1
target_link_libraries(lib2 lib1)
app/CMakeLists.txt:
project(App)
add_executable(app main.cpp some_header.h)
# Add /lib1 and /lib2 to #include search path
include_directories(${Lib1_SOURCE_DIR} ${Lib2_SOURCE_DIR})
# Specify app's dependency on lib2.
# lib2's dependency on lib1 is automatically added.
target_link_libraries(app lib2)
There are plenty of different ways to achieve the same end result here. For a relatively small project, I'd probably just use a single CMakeLists.txt:
cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
project(Test)
add_library(lib1 lib1/lib1.cpp lib1/lib1.h)
add_library(lib2 lib2/lib2.cpp lib2/lib2.h)
add_executable(app app/main.cpp app/some_header.h)
include_directories(${CMAKE_SOURCE_DIR}/lib1 ${CMAKE_SOURCE_DIR}/lib2)
target_link_libraries(lib2 lib1)
target_link_libraries(app lib2)
For further info on the relevant commands and their rationale, run:
cmake --help-command add_subdirectory
cmake --help-command include_directories
cmake --help-command target_link_libraries

Answer 2

Project
CMakeLists.txt
\-lib1
CMakeLists.txt
\- include \ lib1
\- src
\-lib2
CMakeLists.txt
\- include \ lib2
\- src
\-app
CMakeLists.txt
\- src
Suppose dependencies as follow:
lib1 ---> lib2 ---> app
\--------------> app
Something like this:
CMakeLists.txt:
add_subdirectory(lib1)
add_subdirectory(lib2)
add_subdirectory(app)
lib1/CMakeLists.txt:
file(GLOB_RECURSE _HDRS "include/*.hpp")
file(GLOB_RECURSE _SRCS "src/*.[hc]pp")
add_library(lib1 ${_HDRS} ${_SRCS})
#target_link_libraries(lib1)
target_include_directories(lib1 PUBLIC include)
install(TARGETS lib1 DESTINATION lib)
install(FILES ${_HDRS} DESTINATION include/lib1)
lib2/CMakeLists.txt:
file(GLOB_RECURSE _HDRS "include/*.hpp")
file(GLOB_RECURSE _SRCS "src/*.[hc]pp")
add_library(lib2 ${_HDRS} ${_SRCS})
target_link_libraries(lib2 lib1)
target_include_directories(lib2 PUBLIC include)
install(TARGETS lib2 DESTINATION lib)
install(FILES ${_HDRS} DESTINATION include/lib2)
so in lib2/src/file.cpp you could do #include <lib1/header.hpp>
app/CMakeLists.txt:
file(GLOB_RECURSE _SRCS "src/*.[hc]pp")
add_executable(app ${_SRCS})
target_link_libraries(app lib1 lib2)
install(TARGETS app DESTINATION bin)
so in app/src/file.cpp you could do #include <lib1/header.hpp> and #include <lib2/header.hpp>
The magic is target_include_directories which attach the "include" directory to the target, so when linking with it you pull the include directory also ;)