Legacy code with atldbcli and OLEDB does behave differently with C++20

Question

I have a legacy code base where I connect with OLEDB to a MSSQL server.

Now here is the code:

#ifndef _WIN32_WINNT
#define _WIN32_WINNT 0x0A00
#endif

#include <afx.h>
#include <atlbase.h>
#include <atldbcli.h>

// -------------------------------------------------------------------
// OpenWithUDL - connect and open database using an udl file
// -------------------------------------------------------------------
HRESULT OpenWithUDL(CSession& m_session, const CString& pathToUdl)
{
	CDataSource db;
	USES_CONVERSION;

	HRESULT hr = db.OpenFromFileName(T2COLE(pathToUdl));
	if (FAILED(hr)) { return hr; }
	
	CDBPropSet dbinit(DBPROPSET_DBINIT);
	dbinit.AddProperty(DBPROP_INIT_OLEDBSERVICES, (short)DBPROPVAL_OS_ENABLEALL);

	return m_session.Open(db);
}
// -------------------------------------------------------------------
// Connect to database
//
// SetConnStr() or SetUdlStr() must be called first to define the db
// to open - when not set use hardcoded db details for connection
// -------------------------------------------------------------------
HRESULT Open(CSession& m_session, const CString& pathToUdl)
{
	HRESULT hr = CoInitialize(NULL);
	if (FAILED(hr)) { return E_FAIL; }

	return OpenWithUDL(m_session, pathToUdl);
}
// -------------------------------------------------------------------
// Return:	true  db connection ok - session param not NULL
//					false db connection failed
// -------------------------------------------------------------------
bool GetDBCon(const CSession& p_session)
{
	if (p_session.m_spOpenRowset == NULL) { return false; }

	HRESULT hr;
	XACTTRANSINFO pInfo;
	try
	{
		hr = p_session.GetTransactionInfo(&pInfo);
	}
	catch (...)
	{
		return false;
	}
 	return hr == S_OK || hr == XACT_E_NOTRANSACTION;
}

int main()
{
	CSession session;
	CString pathToUdl = "SomePathToUDL";
 	
	HRESULT hr = Open(session, pathToUdl);
	if (FAILED(hr)) { return -1; }

 	try
	{
		if (!GetDBCon(session)) { return -2; }
	}
 	catch (...)
	{
		return -3;
	}
 	
	CString sqlStatement = "SELECT GETDATE();"; // Some select statement

 	CCommand<CDynamicAccessor> d_tab; // <<-- Here is the Problem
	hr = d_tab.Open(session, sqlStatement);
	if (FAILED(hr))
	{
		d_tab.Close();
		return -4;
	}
 	hr = d_tab.MoveFirst(); // <<-- throws exception because ATLASSUME(m_pAccessor != NULL) fails
}

Now the interesting part is, at the process of instantiation of d_tab, CAccessorRowset gets its ctor called. There it registers itself as accessor (see code listing).

///////////////////////////////////////////////////////////////////////////
// CAccessorRowset
template <class TAccessor = CNoAccessor, template <typename T> class TRowset = CRowset>
class CAccessorRowset :
	public TAccessor,
	public TRowset<TAccessor>
{
public:
	template <class _Self = CAccessorRowset>
	CAccessorRowset()
	{
		// Give the rowset a pointer to the accessor
#ifdef _ATL_MODULES
		if constexpr(_Has_m_nAccessors<_Self>::value)
#else // ^^ _ATL_MODULES / !_ATL_MODULES vv
		__if_exists(m_nAccessors)
#endif // _ATL_MODULES
		{
			static_cast<_Self*>(this)->SetAccessor(this); // <<--- register as accessor
		}
	}
//...
};

The provided code at the top throws the exception regardless of the C++ version. But in my legacy code the access to the DB is compiled to a .lib with C++17. Excecuting the calls in main results in registering as accessor, but not so, when it was compiled in C++20. Which to me makes no sense. Does anyone know a solution to that?

Answer 1

Hi, @Schmidiger Kevin Patrick, SPS52-11

From the bug report, Crash in ATL CEnumerator::MoveNext after upgrading to 17.3 toolset with conformance mode.
Please define _ATL_MODULES in preprocessor definition.

Best regards,

Minxin Yu

---

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.