I want to use the new file sets feature of cmake 3.28 since it seems to simplify a lot of steps (installing, exporting). However, I can’t really wrap my head around what exactly is going on behind the scenes. Here’s my project cmake file:

lib_file_loader/CMakeList.txt:

cmake_minimum_required(VERSION 3.28)

project(lib_file_loader VERSION 0.1.0 LANGUAGES CXX)

add_library(lib_file_loader src/file_loader.cpp)
add_library(file_loader::lib_file_loader ALIAS lib_file_loader)


target_sources(lib_file_loader
    PRIVATE
        "src/file_loader.cpp"
    PUBLIC FILE_SET HEADERS BASE_DIRS "src/include" FILES
        "src/include/lib_file_loader/file_loader.h"
)

target_include_directories(lib_file_loader
    PRIVATE FILE_SET HEADERS
    PUBLIC FILE_SET INTERFACE_HEADERS
)

message(STATUS "Interface headers: ${lib_file_loader_INTERFACE_HEADERS}")

This generates ok, however the output of the interface headers is empty:

-- Interface headers:  <--- i expected this to be filled
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: /home/myuser/../file_loader/build

The way I thought this would work was for cmake to detect automatically that I want to create a file set of type HEADERS for PUBLIC headers and it would choose the INTERFACE_HEADERS set for me to populate the files into. This however doesn’t seem to be the case.

So I thought ok maybe I need to specify the fileset name explicitely, so I changed the ?target_sources line to this:

PUBLIC FILE_SET INTERFACE_HEADERS TYPE HEADERS BASE_DIRS "src/include" FILES

But this gives me an error saying that:

CMake Error at lib_file_loader/CMakeLists.txt:9 (target_sources):
  target_sources Non-default file set name must contain only letters,
  numbers, and underscores, and must not start with a capital letter or
  underscore

I’m a bit lost on how to populate the interface header set to retrieve it later for target_include_directories. Things I also hoped for (but seem to be mistaken) are:

• target_include_directories will automatically know for file sets of HEADERS type to go into either PRIVATE or PUBLIC category without respecifying.
• BASE_DIRS (what is it for anyway? I can’t find anything in the documentation) to allow header files to automatically be globbed up in the said directory so I don’t even have to specify them.

I like to keep my cmake minimal and thought file sets look good enough, but I don’t seem to understand them properly. Can someone shed some light on the mechanics on how to use them?