As described in QTCREATORBUG-17861: QtCrator 4.2 does not list header-only implementations in the project view if they are not mentioned in any CMakeLists.txt file.

New in QtCreator 4.3 beta 1: header files are listed in the project view even if they are not mentioned in the cmake files (as long as "hide generated files" is not checked).

A workaround for QtCreator 4.2 is to create a dummy target that collects all header files:

file(GLOB_RECURSE ALL_HEADER_FILES "${CMAKE_CURRENT_SOURCE_DIR}/*.hpp")
add_library(dummy_target_with_all_headers EXCLUDE_FROM_ALL ${ALL_HEADER_FILES})
target_link_libraries(dummy_target_with_all_headers)
set_target_properties(dummy_target_with_all_headers PROPERTIES LINKER_LANGUAGE CXX)

Opening the project in QtCreator 4.3 with such a dummy target leads to doubled header files in the project view (if "hide generated files" is not checked).