Export custom library which uses Qt via CMake for use in another CMake project (Windows, Mingw-w64)

Question

A bit of background on what I am trying to achieve: I already have a project that I have developed in CMake (it is collection of CMake projects: basically a state machine with a few helper libraries). Now, I want to develop a GUI in Qt that this state machine can control (and make it do stuff). However, for the life of me, I cannot figure out how to properly export the library which has the Qt classes and functions to another CMake package's executable. I am on Windows, and using the Mingw-w64 compiler and make program.

I was experimenting with 2 simple CMake projects, one which has a simple Qt form (which I just copied from and modified this repository and the corresponding youtube video given in the README), and the other which has the executable. I was trying to link the first project with the second project, however, whenever I build the autogenerated makefile, it fails in the end, saying that symbols are undefined (For example undefined reference to run_stuff::run_stuff()'). I apologize if the files are dirty, I have been trying out various stuff to no avail.

Things I have tried:

• I have followed the answer here very carefully.

• I have tried setting CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS to ON, as suggested by many sites (like this one) for running DLLs

• I have tried making the library static (hence doing away with the need for DLLs)

• I have tried setting the target_include_dirs from private to public and also commenting out that line. (not sure what that does, but okay)

However, no matter what I do, I always end with linking errors in the end, and my executable in the second project is not able to access any functions from the Qt project. Any suggestions on what I should try next? Would be really helpful if you could point out errors I am making in my CMakeLists.txt files (once again apologies for the messy code and CMakeLists)

My project CMake file looks like this:

cmake_minimum_required(VERSION 3.14)

# if (WIN32)
#   set(CMAKE_SHARED_LIBRARY_PREFIX "")

# endif ()

set(CMAKE_MAKE_PROGRAM $ENV{MAKE})
set(CMAKE_CONFIGURATION_TYPES "Release;RelWithDebInfo" CACHE STRING "" FORCE)
set(CMAKE_SUPPORT_WINDOWS_EXPORT_ALL_SYMBOLS ON)
set(WINDOWS_EXPORT_ALL_SYMBOLS ON)

add_subdirectory(QT6CMake)
add_subdirectory(Exec)

The library CMake file:

cmake_minimum_required(VERSION 3.14)

if (WIN32)
    project(MY_PROJECT LANGUAGES CXX)
elseif(UNIX)
    project(MY_PROJECT)
endif()

set(CMAKE_CONFIGURATION_TYPES "Release;RelWithDebInfo" CACHE STRING "" FORCE)

#======================= INCLUSION OF Qt =======================#
set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTOUIC ON)
set(CMAKE_PREFIX_PATH $ENV{QTDIR})
find_package(Qt6Core REQUIRED)
find_package(Qt6Widgets REQUIRED)

#=================== INCLUSION OF Project Files ====================#
set(FORMS_DIR "${CMAKE_SOURCE_DIR}/forms")
set(INCLUDE_DIR "${CMAKE_SOURCE_DIR}/include")
set(SOURCE_DIR "${CMAKE_SOURCE_DIR}/src")

include_directories(${FORMS_DIR})
include_directories(${INCLUDE_DIR})
include_directories(${SOURCE_DIR})

file(GLOB_RECURSE SOURCES
    "${FORMS_DIR}/*.ui"
    "${FORMS_DIR}/*.qrc"
    "${INCLUDE_DIR}/*.h"
    "${SOURCE_DIR}/*.cpp"
)

#=================== SETUP EXECTUABLE ====================#
# Enable debug logging on RELWITHDEBINFO configuration
set_property(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS
    $<$<CONFIG:RELWITHDEBINFO>:QT_MESSAGELOGCONTEXT>
)

# Add the forms directory to the AUTOUIC search paths
set(CMAKE_AUTOUIC_SEARCH_PATHS ${CMAKE_AUTOUIC_SEARCH_PATHS} ${FORMS_DIR})

# Add the executable
if (WIN32) 
    add_executable(MY_PROJECT WIN32 ${SOURCES})
elseif(UNIX)
    add_executable(MY_PROJECT ${SOURCES})
endif()

# Add the target includes for MY_PROJECT 
target_include_directories(MY_PROJECT PRIVATE ${FORMS_DIR})
target_include_directories(MY_PROJECT PRIVATE ${INCLUDE_DIR})
target_include_directories(MY_PROJECT PRIVATE ${SOURCE_DIR})

#===================== LINKING LIBRARIES =======================#
target_link_libraries(MY_PROJECT Qt6::Widgets)

The Exec CMake file:

cmake_minimum_required(VERSION 3.14)
project(somexec)

add_definitions(${MY_PROJECT_DEFINITIONS})
include_directories(${MY_PROJECT_INCLUDE_DIRS})

if (WIN32) 
    add_executable(${PROJECT_NAME} WIN32 main.cpp)
elseif(UNIX)
    add_executable(${PROJECT_NAME} main.cpp)
endif()

target_link_libraries(${PROJECT_NAME} MY_PROJECT)

Here is the codebase.

EDIT: Finally the code seems to be working, courtesy of Guillaume Racicot's multiple edits and fixes. I am going to leave this repository public in case anyone wants to see the codebase. Also, right now, I don't understand all the CMake commands that he has used, will try to understand those as well