[fdo-internals] [Select] Empty reader or NULL?

[Traian Stanev]

I verified SdfSelect and it throws a class-not-found exception if the class is not in the schema. So yes, an exception seems like the consistent thing to do.
 
Traian

	-----Original Message----- 
	From: fdo-internals-bounces at lists.osgeo.org on behalf of Greg Boone 
	Sent: Mon 3/19/2007 10:07 PM 
	To: FDO Internals Mail List 
	Cc: 
	Subject: RE: [fdo-internals] [Select] Empty reader or NULL?
	
	

	In this specific case, I would image that an exception would be thrown
	stating that 'class non-existing-feature-class-name does not exist'.
	
	In other cases, where no valid data is available for return, but an
	exception is not warranted, I would recommend not returning NULL. I
	recommend returning an instantiated Reader. When the user calls
	ReadNext(), it will return false.
	
	Greg
	
	-----Original Message-----
	From: fdo-internals-bounces at lists.osgeo.org
	[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Mateusz
	Loskot
	Sent: Monday, March 19, 2007 8:43 PM
	To: fdo-internals
	Subject: [fdo-internals] [Select] Empty reader or NULL?
	
	Hi,
	
	I'd like to ask about semantic details of FdoISelect::Execute()
	operation, especially in DBMS provider.
	
	For instance, if a feature class is directly mapped to table in a
	database.
	Let's imagine the code below is called by a client:
	
	
	// Connection is Open
	FdoPtr<FdoIConnection> conn;
	
	FdoPtr<FdoISelect> cmd;
	cmd = static_cast<FdoISelect*>(
	    conn->CreateCommand(FdoCommandType_Select));
	assert(0 != cmd);
	
	// NOTE: Querying non-existing class
	cmd->SetFeatureClassName(L"non-existing-feature-class-name");
	
	// XXX - What Execute() is supposed to return here?
	FdoPtr<FdoIFeatureReader> reader = cmd->Execute();
	
	
	My question is as in line marked with XXX:
	If a client queries for a feature class that does not
	exist (a non-existing table), what the Execute() operation is supposed
	to return?
	
	Is it safe to return NULL pointer to a reader?
	
	Or it should return a kind of empty (terminating) reader, however here I
	have no idea how to achieve this kind of feature.
	
	Or an exception is thrown from Execute()?
	
	Cheers
	--
	Mateusz Loskot
	http://mateusz.loskot.net <http://mateusz.loskot.net/> 
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