Can I make a library from multiple targets? - c++

Question

I'm trying to learn cmake and have started converting an old make project over to cmake. Here is a simplified version of the directory structure I now have:
CMakeLists.txt
src/
CMakeLists.txt
main.cpp
core/
CMakeLists.txt
/sourcecode, other cmakes, etc.
test/
CMakeLists.txt
someTest.cpp
Currently, in my root CMakeLists.txt file I simply have this:
cmake_minimum_required(VERSION 2.8)
project(all)
add_subdirectory(src)
add_subdirectory(test)
What I wanted to do, was have a library created by core/CMakeLists.txt that can be used by both src/CMakeLists.txt to build the main executable, but also loaded by test/CMakeLists to build the unit tests.
So my src/core/CMakeLists.txt file currently looks sort of like this:
cmake_minimum_required(VERSION 2.8)
project(core)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wpedantic -Wreorder -DBOOST_TEST_DYN_LINK -DBOOST_LOG_DYN_LINK ")
#some other directories in my core code:
add_subdirectory(display)
add_subdirectory(training)
add_subdirectory(utility)
#some packages I use...
find_package(Boost 1.55.0
COMPONENTS
log
program_options
serialization
thread
system
filesystem
REQUIRED)
find_package(GLUT REQUIRED)
find_package(OpenGL REQUIRED)
find_package(Eigen3 REQUIRED)
include_directories(
${PROJECT_SOURCE_DIR}
${EIGEN3_INCLUDE_DIR})
target_link_libraries(core
display
training
utility
${Boost_LIBRARIES}
${OPENGL_LIBRARIES}
${GLUT_LIBRARY}
${OpenMP_LIBRARIES})
So the idea is that I now have a core target I can simply link against to run my tests, and everything should work. However, when I try to build main, for example, I get:
Cannot specify link libraries for target "core" which is not built by this
project.
I thought this might be because core doesn't have a add_library command, but if I add add_library(core) I get this error:
You have called ADD_LIBRARY for library core without any source files. This typically indicates a problem with your CMakeLists.txt file
But I don't want to add any source files; I just want this target to link the targets in the core directory and produce a target I can link against from test.
Clearly I'm missing some core knowledge here, either with cmake or the toolchain itself. Help is appreciated :)

Answer 1

If you only want to create a core target without source files, you need to declare it like an INTERFACE target. So, try to add the following code to your src/core/CMakeLists.txt:
cmake_minimum_required(VERSION 3.0) # REQUIRED 3.x.x version
project(core)
...
# Here declare your core_interface target
add_library(core_interface INTERFACE)
target_link_libraries(core_interface INTERFACE
display
training
utility
${Boost_LIBRARIES}
${OPENGL_LIBRARIES}
${GLUT_LIBRARY}
${OpenMP_LIBRARIES})
As you can see, if you make this, you'll need to upgrade your CMake installed version.
Then, you'll build your tests or any executable, linking with this interface target directly:
add_executable(main main.cpp)
target_link_libraries(main core_interface)