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!
