cmake include header only library

Question

I want to include spdlog into one of my project. It is a header only library. The project that i am building is using cmake. Currently i am using

include_directories('src/logger/spdlog/')

in cmake and including the library as

#include <spdlog/spdlog.h>

in logs.h inside logger folder. I am getting fatal error no such file or directory. What is the correct way to include the same library in my application.

Answer 1

First, don't use single quotes ' but double quotes " or just plain strings if they don't contain spaces.

I would advice you to use find_path instead of adding includes directly. There you can add PATH_SUFFIXES. You get a message during configuration if the header is not found, which makes it easier to spot errors.
Documentation: https://cmake.org/cmake/help/v3.6/command/find_path.html

Answer 2

You are probably off one directory. Try either

 include_directories("src/logger")

in the CMakeLists.txt, or

 #include <spdlog.h>

in the source code.

Answer 3

Cmake provides interface library specifically for "header-only library". I would suggest to do the following:

1. Create a file structure like the following

third-party\spdlog\include

2. Git clone the spdlog repository into the include directory with the name spdlog
3. Create third-party\spdlog\CMakeLists.txt with the following content

find_package(Threads REQUIRED)
add_library(${TARGET_LOGGER} INTERFACE)
# add_library(spdlog::spdlog_header_only ALIAS ${TARGET_LOGGER})
target_include_directories(${TARGET_LOGGER} INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
                                                        "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
target_link_libraries(${TARGET_LOGGER} INTERFACE Threads::Threads)

4. Define TARGET_LOGGER in the CMakeLists.txt of your project. Link this target to your project with target_link_libraries(${TARGET_PROJECT} LINK_PUBLIC ${TARGET_LOGGER}). Also don't forget to

add_subdirectory(
    third-party\spdlog
  )

Then, in your project, you can use #include "spdlog/spdlog.h" to include the library