how to include boost headers in own header-only library - c++

Question

i am using Jetbrains CLion 2017.3 and the bundled CMake version 3.9.6 with mingw64 5.0 version/g++ 7.1.
Although reading the "Mastering CMake" ( i am new to CMake !) i have many difficulties to understand the basics. Since 3 days i am searching for a CMake solution to create a own header-only library that uses the boost (1.66.0 ) libraries.
Using my CMakeLists.txt results in finding the boost libraries, but i cannot include boost headers in a header file from my current source directory.
My current source diretory contains the "CMakeLists.txt" and the header file
"test_boost.h".
If i try to include boost headers in the header file "test_boost.h", boost headers cannot be found !
What i am doing wrong ?
My CMakeLists.txt :
cmake_minimum_required(VERSION 3.9)
project(headerOnlyLib1)
set(CMAKE_CXX_STANDARD 11)
set(ENV{BOOST_ROOT} "C:/dev/boost/mingw/boost_1_66_0/boost")
set(Boost_USE_STATIC_LIBS ON) # only find static libs
set(Boost_USE_MULTITHREADED ON)
set(Boost_USE_STATIC_RUNTIME OFF)
find_package(BOOST 1.66 REQUIRED)
IF (Boost_FOUND)
message(STATUS "BOOST FOUND !")
ELSE()
message(STATUS "BOOST NOT Found !")
endif()
add_library(headerOnlyLib INTERFACE)
target_include_directories(headerOnlyLib INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
target_include_directories(headerOnlyLib INTERFACE ${Boost_INCLUDE_DIRS})
target_link_libraries(headerOnlyLib ${Boost_LIBRARIES})

Answer 1

Short answer: You can't.
A "header-only library" is just that, one or more headers, only. It's not something that is linked or really stand-alone.
If your header-only library have dependencies, then the users of your library also have those dependencies and need to include them in their own build.

Answer 2

I think that you can, but you need to be more specific in defining your boost dependencies.
For example, the CMakeLists.txt file here depends upon boost::system for boost::asio. The dependency is defined as follows:
find_package(Boost REQUIRED COMPONENTS system)
if(Boost_FOUND)
target_include_directories(${PROJECT_NAME} PRIVATE ${Boost_INCLUDE_DIRS})
# Boost::asio is header only but it requires Boost::system
target_link_libraries(${PROJECT_NAME} INTERFACE Boost::system)
.
.
.
endif(Boost_FOUND)
In your case, the target is Boost::boost for header-only dependencies, see FindBoost. So the relevant part becomes:
find_package(Boost 1.66 REQUIRED COMPONENTS boost)
IF (Boost_FOUND)
message(STATUS "BOOST FOUND !")
target_include_directories(headerOnlyLib INTERFACE ${Boost_INCLUDE_DIRS})
ELSE()
message(STATUS "BOOST NOT Found !")
endif()
I recommend watching Daniel Pfeifer's talk at C++ Now 2017 for more information.
A lot has changed since "Mastering CMake" was written...