Problems with Delphi DLL IStream parameter calling from C++ - c++

Question

I have DLL written on Delphi. I have only DLL without header file, so I load it dynamically. (to C++ project)
HMODULE hLib = LoadLibrary(L"LibName.dll");
if (!hLib) {
//Throw error
}
DLL provides functions:
function DataToFile(AddressName: PChar; Request: PChar;
RequestSize: integer; ResultFile: PChar;
ErrorBuf: PChar; ErrorBufSize: integer):BOOL;stdcall;
function DataToStream(AddressName: PChar; Request: PChar;
RequestSize: integer; ResultStream: IStream;
ErrorBuf: PChar; ErrorBufSize: integer):BOOL;stdcall;
My Visual Studio Code (C++):
typedef bool(__stdcall* f_DataToFile)(
char*, //AddressName: PChar
char*, //Request: PChar
int, //RequestSize: integer
char*, //FileName: PChar
char*, //ErrorBuf: PChar
int); //ErrorBufSize: integer);
typedef bool(__stdcall* f_DataToStream)(
char*, //AddressName: PChar
char*, //Request: PChar
int, //RequestSize: integer
std::istream &, //ResultStream: IStream
char*, //ErrorBuf: PChar
int); //ErrorBufSize: integer);
...
Get funct. address:
//load DataToFile
f_DataToFile DataToFile = (f_DataToFile) GetProcAddress(hLib, "DataToFile");
if (!DataToFile) { //throw error
}
//load DataToStream
f_DataToStream DataToStream = (f_DataToStream) GetProcAddress(hLib, "DataToStream");
if (!DataToStream) { //throw error
}
...
Set data:
char* AddressName = _strdup("127.0.0.1:1234"); //AddressName: PChar
char* Request = _strdup("<?xml request... >"); //Request: PChar
int RequestSize = strlen(Request); //RequestSize: integer
char* ResultFile = _strdup("exportpath\\output.xml"); //FileName: PChar
char* ErrorBuf = new char[255]; //ErrorBuf: PChar
int ErrorBufSize = 255; //ErrorBufSize: integer);
std::filebuf(buffer);
std::istream ResultStream(&buffer);
...
First function working correctly
bool reesult1= (DataToFile)(AddressName, Request, RequestSize, ResultFile, ErrorBuf, ErrorBufSize);
...
I have problems with second function execution -
bool reesult2= (DataToStream)(AddressName, Request, RequestSize, ResultStream, ErrorBuf, ErrorBufSize);
It is compiling, but gives Access Violoation on run.
Can someone help me to get - how to correctly work with IStream data type from (Delphi)?
When i declare ResultStream as nullptr pointer and call DataToStream function with to incorrect connection address, function return "Connection error" - so it is imported correctly and main question is returning IStream from function.

Answer 1

Your translation of IStream is not correct. The DLL is using the COM IStream interface. You can't replace that with C++'s std::istream class. You need to use a COM object that implements the IStream interface. To access a file as an IStream, you can use the Win32 SHCreateStreamOnFileEx() function, for instance.

Answer 2

Thanks to Remy Lebeau!
HRESULT GetFileStream(
_In_ LPCWSTR fullFileName,
_Outptr_ IStream** stream)
{
HRESULT hr = S_OK;
// Create stream for writing the file
if (SUCCEEDED(hr))
{
hr = SHCreateStreamOnFileEx(
fullFileName,
STGM_CREATE | STGM_WRITE | STGM_SHARE_EXCLUSIVE,
0, // default file attributes
TRUE, // create file if it does not exist
NULL, // no template
stream);
}
return hr;
}
in typedef:
IStream *, //ResultStream: IStream
..
LPCWSTR filename = L"path\\to\\file.txt";
IStream* ResultStream = NULL;
HRESULT hr = GetFileStream(filename, &ResultStream);
..
bool result = (CallRK7XMLRPCToStream)(AddressName, Request, RequestSize, ResultStream, ErrorBuf, ErrorBufSize);
here is a good example of using SHCreateStreamOnFileEx:
https://github.com/Microsoft/Windows-classic-samples/blob/master/Samples/AppxPackingCreateBundle/cpp/CreateBundle.cpp