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:

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.