library test_ws;

uses
  Windows, SysUtils, Vcl.Dialogs;

function GetLastErrText_O(out str: WideString): BOOL; stdcall;
begin
  str := 'This is the result of GetLastErrText_O :: zЭто результат работы GetLastErrText_O.:;';
  Result := True;
end;

exports
  GetLastErrText_O;

begin
end.

#include <iostream>
#include <windows.h>

using namespace std;

int main()
{
    HMODULE lib = LoadLibrary("test_ws_o.dll");

    typedef BOOL (__stdcall *Func)(BSTR &pstr);
    Func GetLastErrText_O = (Func) GetProcAddress(lib, "GetLastErrText_O");

    BSTR bstr;
    BOOL flg;

    flg = GetLastErrText_O(bstr);
    std::wcout << ".a." << bstr << ".z.";  // ЗДЕСЬ ВЫВОДИТ: ".a.This is the result of GetLastErrText_O :: z"
    std::cout <<"\n";
    SysFreeString(bstr);

    return 0;
}

_setmode(_fileno(stdout), _O_U16TEXT);