I need to create a Flutter project for windows that can use a FEIG device for NFC scanning. The only solution I have found so far is to use Flutter FFI (Foreign Function Interface) to compile C++ files and call their methods in Dart. The C++ files will call methods from the Feig libraries, and a CMakeLists.txt file will be used to compile the C++ code into a DLL (Dynamic Link Library). Flutter FFI can then read the methods from the DLL, allowing me to access and use FEIG classes and methods in my Flutter application.

This is a demo where I want to check if the Feig device is connected to the USB port using

Directory Struct:

flutter_feig_app_demo_1/
├── build/
│   ├── CMakesLists.txt
│   ├── cpp_lib.cpp
|   ├── MyDLL.vcxproj
|   └── Release/
|       └── MyDLL.dll
└── lib/
    ├── main.dart
    ├── native_lib.dart
    ├── lib
    |   └── release
    |       └── feiglib.lib
    └── include
        └── fedm
            ├── Reader.h
            └── Manager.h

CMakeLists.txt:

cmake_minimum_required(VERSION 3.14)
project(flutter_feig_app_demo_1 LANGUAGES CXX)

set(SOURCES cpp_lib.cpp)
include_directories(../lib/include/fedm)

# Specify the library directories
link_directories(../lib/release)

# Add the shared library (DLL)
add_library(MyDLL SHARED ${SOURCES})

target_link_libraries(MyDLL PRIVATE ../lib/release/feiglib)

# Set the output directory for the DLL
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
# Enable Windows export symbols
if(WIN32)
    set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
endif()

cpp_lib file:

#include <iostream>
#include "Reader.h"
#include "Manager.h"



extern "C" 
{

    using namespace FEDM;
    using namespace std;
    

    int scanUSB()
    { 
        return Manager::reader();
    }
}

native_lib.dart

import 'dart:ffi';
import 'dart:io';

final DynamicLibrary nativelib = DynamicLibrary.open("lib/build/Release/MyDLL.dll");

typedef AddFunc = Int32 Function();
typedef Add = int Function();

final Add add = nativelib.lookupFunction<AddFunc,Add>('scanUSB');

int isDiscovered(){
  return add();
}

main:

import 'package:flutter/material.dart';
import 'native_lib.dart';
void main() {
  runApp(const MainApp());
}

class MainApp extends StatelessWidget {
  const MainApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text('Hello World! ${isDiscovered()}'),
        ),
      ),
    );
  }
}

Error:

cpp_lib.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: static unsigned __int64 __cdecl FEDM::Manager::reader(void)" (__imp_?reader@Manager@FEDM@@SA_KXZ) referenced in function scanUSB [root\build\MyDLL.vcxproj]
root\build\Release\MyDLL.dll : fatal error LNK1120: 1 unresolved externals [root/build\MyDLL.vcxproj]