dll возвращающая текстовую строку

// MathLibrary.h - Contains declarations of math functions
#pragma once
 
#ifdef MATHLIBRARY_EXPORTS
#define MATHLIBRARY_API __declspec(dllexport)
#else
#define MATHLIBRARY_API __declspec(dllimport)
#endif
 
extern "C" MATHLIBRARY_API string GetHWID();

// MathLibrary.cpp : Defines the exported functions for the DLL.
#include "pch.h" // use stdafx.h in Visual Studio 2017 and earlier
#include <utility>
#include <limits.h>
#include "MathLibrary.h"
#include <windows.h>
#include <string>
#include <iostream>
#include <sstream>
#include <fstream>
 
using namespace std;
 
string GetHWID()
{
    string initstring;
    initstring = "Error : string is null";
    UCHAR szFileSys[255], szVolNameBuff[255];
    DWORD dwSerial;
    DWORD dwMFL;
    DWORD dwSysFlags;
    int error = 0;
 
    bool fail = GetVolumeInformationA("C:\\", (LPSTR)szVolNameBuff, 255, &dwSerial, &dwMFL, &dwSysFlags, (LPSTR)szFileSys, 255);
    if (!fail) {
        initstring = "Error : Not elevated (please run this with admin rights)";
        return initstring;
    }
 
    stringstream hwidstream;
    hwidstream << hex << dwSerial;
 
    string HWID = hwidstream.str();
 
    for (int i = 0; i < HWID.length(); i++)
        HWID[i] += 6; // (you can change this to shift as much as you want)
 
    return HWID;
}

// MathLibrary.h - Contains declarations of math functions
#pragma once
#include <wtypes.h>
//#include <string>
 
#ifdef MATHLIBRARY_EXPORTS
#define MATHLIBRARY_API __declspec(dllexport)
#else
#define MATHLIBRARY_API __declspec(dllimport)
#endif
 
extern "C" MATHLIBRARY_API void fibonacci_init(
    const unsigned long long a, const unsigned long long b);
 
extern "C" MATHLIBRARY_API bool fibonacci_next();
 
extern "C" MATHLIBRARY_API unsigned long long fibonacci_current();
 
extern "C" MATHLIBRARY_API unsigned fibonacci_index();
 
extern "C" MATHLIBRARY_API BSTR GetHWID();