Why add header files into ADD_LIBRARY/ADD_EXECUTABLE command in CMake - cmake

Question

I had a project which uses CMake as build tool and made a simple template for me and my collegues to use. As I searched for best and easy to use practices online, I've came across different approaches to make a library.
In this template, I've listed header files and source files in two seperate variables, and I'm not passing the headers to add_library command - just sources. And then I use set_target_properties with PUBLIC_HEADER variable to give the header-file list.
So far it seems to work, but I wonder if I'm making thing unnecessarily complex. Some people online give header files to add_library command as well and doesn't even use set_target_properties and such.
In short:
should we include header files to add_library or should we not (as a best practice)? And impacts of the two usage.
what is purpose being served by adding headers in the add_library/add_executable? As they seem working even without it (seems forward declaration and symbols only). confirm on understanding please.
(Here is the template I'm talking about:)
cmake_minimum_required(VERSION 3.1.0)
project(lae CXX C)
set(CMAKE_CXX_STANDARD 14)
include_directories(
${CMAKE_CURRENT_SOURCE_DIR}
)
set(SOURCE_FILES
...
)
set(HEADER_FILES
...
)
set( PRIVATE_HEADER_FILES
...
)
add_library(${PROJECT_NAME} SHARED ${SOURCE_FILES} )
set( REQUIRED_LIBRARIES
...
)
target_link_libraries(${PROJECT_NAME} ${REQUIRED_LIBRARIES} )
SET_TARGET_PROPERTIES(
${PROJECT_NAME}
PROPERTIES
FRAMEWORK ON
SOVERSION 0
VERSION 0.1.0
PUBLIC_HEADER "${HEADER_FILES}"
PRIVATE_HEADER "${PRIVATE_HEADER_FILES}"
ARCHIVE_OUTPUT_DIRECTORY "lib"
LIBRARY_OUTPUT_DIRECTORY "lib"
OUTPUT_NAME ${PROJECT_NAME}
)

Answer 1

In our projects we use a "simple" way of yours - add_library with both headers and sources.
If you add only sources, then you won't see headers in IDE-generated project.
However, when installing, we have to do it like that, using two install commands:
install(TARGETS library_name
LIBRARY DESTINATION lib)
install(FILES ${PUBLIC_HEADERS}
DESTINATION include/library_name)
If you want to do it as a single command, you can use set_target_properties with PUBLIC_HEADER, as you suggested.
Then, this kind of install is possible:
install(TARGETS library_name
LIBRARY DESTINATION lib
PUBLIC_HEADER DESTINATION include/library_name)
Choose the one you like the most and stick to it.