Adding header-only dependencies with CMake

Question

I have a simple project which requires three header-only libraries in order to compile: websocketpp, spdlog and nlohmann/json.

The project structure looks like this:

└── src
    ├── app
    │   ├── CMakeLists.txt
    │   ├── src
    │   └── test
    ├── CMakeLists.txt
    ├── core
    │   ├── CMakeLists.txt
    │   ├── include
    │   ├── src
    │   └── test
    └── vendor
        ├── install.cmake
        ├── nlohmann_json
        ├── spdlog
        └── websocketpp

The root CMakeLists.txt is as follows:

cmake_minimum_required(VERSION 3.6.1 FATAL_ERROR)

..

# External 3rd party libs that we include

include(vendor/install.cmake)

add_subdirectory(core)
add_subdirectory(app)

The idea is basically that each subdirectory is a library (e.g. core), and app "aggregates" all of them. Each library (e.g. core) is built like this (core/CMakeLists.txt):

project(foo-core VERSION 0.1 LANGUAGES CXX)
add_library(foo-core
  src/foobar/foobar.cc
  src/foobaz/baz.cc)

target_include_directories(foo-core PUBLIC
  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
  $<INSTALL_INTERFACE:include>
  PRIVATE src)

target_link_libraries(foo-core websocketpp spdlog) # <- see here, using spdlog & websocketpp

# 'make install' to the correct location

install(TARGETS foo-core EXPORT FooCoreConfig
  ARCHIVE  DESTINATION lib
  LIBRARY  DESTINATION lib
  RUNTIME  DESTINATION bin)
install(DIRECTORY include/ DESTINATION include)

install(EXPORT FooCoreConfig DESTINATION share/FooCore/cmake)

export(TARGETS foo-core FILE FooCoreConfig.cmake)

Notice how I link the dependencies (which are header-only libraries!). This is how I fetch them (vendor/install.cmake):

# spdlog

if((NOT SPDLOG_INCLUDE_DIR) OR (NOT EXISTS ${SPDLOG_INCLUDE_DIR}))
  message("Unable to find spdlog, cloning...")

  execute_process(COMMAND git submodule update --init -- vendor/spdlog
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})

  set(SPDLOG_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/vendor/spdlog/include/
    CACHE PATH "spdlog include directory")

  install(DIRECTORY ${SPDLOG_INCLUDE_DIR}/spdlog DESTINATION include)

  # Setup a target

  add_library(spdlog INTERFACE)
  target_include_directories(spdlog INTERFACE
    $<BUILD_INTERFACE:${SPDLOG_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)

  install(TARGETS spdlog EXPORT spdlog DESTINATION include)
endif()

# websocketpp

if((NOT WEBSOCKETPP_INCLUDE_DIR) OR (NOT EXISTS ${WEBSOCKETPP_INCLUDE_DIR}))
  message("Unable to find websocketpp, cloning...")

  execute_process(COMMAND git submodule update --init -- vendor/websocketpp
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})

  set(WEBSOCKETPP_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/vendor/websocketpp/
    CACHE PATH "websocketpp include directory")

  install(DIRECTORY ${WEBSOCKETPP_INCLUDE_DIR}/websocketpp DESTINATION include)

  # Setup a target

  add_library(websocketpp INTERFACE)
  target_include_directories(websocketpp INTERFACE
    $<BUILD_INTERFACE:${WEBSOCKETPP_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)

  install(TARGETS websocketpp EXPORT websocketpp DESTINATION include)
endif()

# nlohmann/json

if((NOT NLOHMANN_JSON_INCLUDE_DIR) OR (NOT EXISTS ${NLOHMANN_JSON_INCLUDE_DIR}))    
  message("Unable to find nlohmann/json, cloning...")

  execute_process(COMMAND git submodule update --init -- vendor/nlohmann_json
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})

  set(NLOHMANN_JSON_INCLUDE_DIR
    ${CMAKE_CURRENT_SOURCE_DIR}/vendor/nlohmann_json/src/
    CACHE PATH "nlohmann/json include directory")

  install(FILES ${NLOHMANN_JSON_INCLUDE_DIR}/json.hpp DESTINATION include)

  # Setup a target

  add_library(nlohmann_json INTERFACE )
  target_include_directories(nlohmann_json INTERFACE
    $<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_DIR}>
    $<INSTALL_INTERFACE:include>)

  install(TARGETS nlohmann_json EXPORT nlohmann_json DESTINATION include)
endif()

So far so good: you can see the dependencies are fetched as git submodules, which thankfully makes it easier to manage them. However, when I compile my project with mkdir build && cd build && cmake ../src, I have the following errors:

CMake Error: install(EXPORT FooCoreConfig ...) includes target foo-core which requires target websocketpp that is not in the export set.

CMake Error: install(EXPORT FooCoreConfig ...) includes target foo-core which requires target spdlog that is not in the export set.

Including the headers e.g. #include <spdlog/spdlog.h> or #include <nlohmann/json.hpp> produces error saying that the header is not found.

Truth be told, I am not so comfortable with CMake and I have spent the past two days debugging this. It might be something really simple, but I can't figure how to achieve it. In reality, one would just pass -I as a compiler flag to use the libraries like I want, but the CMake abstraction seems to confuse me. I would be very glad if someone could explain why this doesn't work and, hopefully, what's the correct way of including these libraries to my project. Thanks in advance!

Answer 1

Just as you're said: you didn't install your targets in the export set. In other words, you're missing an install(EXPORT ... line for your header-only targets. For example, considering your header-only library websocketpp, you should have:

add_library(websocketpp INTERFACE)
target_include_directories(websocketpp INTERFACE
  $<BUILD_INTERFACE:${WEBSOCKETPP_INCLUDE_DIR}>
  $<INSTALL_INTERFACE:include>)

install(TARGETS websocketpp EXPORT websocketpp-config DESTINATION include)

# here is the missing line:

install(EXPORT websocketpp-config DESTINATION share/websocketpp/cmake)

Same goes for the other libraries.