How to require a header-only library without binary in cmake? - c++

Question

I created a header-only library, which is installed with cmake using
add_library(mylib INTERFACE)
and exported targets.
This works fine, when I use find_package(mylib REQUIRED) and target_link_library in the cmake file of another library (mylib2), which includes the headers.
But when I link against mylib2 using exported targets and target_link_library, the generated VS files contain mylib.lib in the link libraries.
When I look for the string mylib.lib in the generated files of all three projects, only the visual studio files contain this, none of the PackageConfig or PackageTargets files contain the filename.
The header-library uses exported targets this in its CMakeLists.txt
add_library(mylib INTERFACE)
target_compile_definitions(mylib INTERFACE -D_USE_MATH_DEFINES)
target_link_libraries(mylib INTERFACE somelibraries)
target_include_directories(mylib INTERFACE
$<INSTALL_INTERFACE:include>
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
)
configure_package_config_file(mylibConfig.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/mylibConfig.cmake INSTALL_DESTINATION ${CMAKE_INSTALL_PREFIX})
install(TARGETS mylib EXPORT mylib-targets)
install(EXPORT mylib-targets FILE mylibTargets.cmake DESTINATION ${CMAKE_INSTALL_PREFIX})
install(FILES ${CMAKE_CURRENT_BINARY_DIR}/mylibConfig.cmake DESTINATION ${CMAKE_INSTALL_PREFIX})
install(DIRECTORY ./ DESTINATION include FILES_MATCHING PATTERN "*.h")
The second library just uses
find_package(mylib REQUIRED)
add_library(mylib2 source.cpp)
target_link_libraries(mylib2 mylib)
# some stuff to export targets, similar to the first lib
And the final project uses
add_binary(myproject source.cpp)
set(LINK_LIBRARIES mylib2)
target_link_libraries(myproject ${LINK_LIBRARIES}) # mylib2.lib is added to the project.
add_binary(myproject2 source.cpp)
target_link_libraries(myproject mylib2) # only the include paths are added.
The problem only happens when the libraries are set from a variable.
At which point of the process does cmake add the library filename to the link-libraries? Shouldn't it inherit the INTERFACE property?

Answer 1

To include a library, you usually have to include header files and link some object files.
With that said, if said library is header-only, you only need to perform include_directories(<library-include-folder>) and you're set to go. Since normally your library shouldn't be compiled into an .lib file.
In your case you probably have to do: (assuming mylib is header-only)
find_package(mylib REQUIRED)
# Provides MYLIB_INCLUDE_DIR to include
# The line above also provides a MYLIB_LIBRARIES, which for header-only libraries should be empty
include_directories(${MYLIB_INCLUDE_DIR})
add_executable(main main.cpp)
target_link_libraries(main ${MYLIB_LIBRARIES}) # This can be onmitted since there is nothing to link