ArcGIS Engine Developer Guide

C++ APPLICATION PROGRAMMING INTERFACE
Replicating the functionality of instanceof (Java) or TypeOf (Visual
Basic)
It is common to have an interface pointer that could point to one of several
coclasses. You can find out more information about the coclass by attempting to
QI to other interfaces using if/else logic. For example, both the RasterDataset and
FeatureDataset coclasses implement IDataset. If you are passed IDataset as a function
parameter, you can determine which coclass the IDataset references as follows:
void Foo (IDataset *pDataset)
{
IFeatureDatasetPtr ipFeatureDataset(pDataset);
if (ipFeatureDataset != 0)
{
// Use IFeatureDataset methods.
}
else
{
IRasterDataset2Ptr ipRasterDataset(pDataset);
if (ipRasterDataset!= 0)
{
// Use IRasterDataset2 methods.
}
}
}
Raw pointers in function signatures
Rather than having a smart pointer in the function signature, consider using a raw
pointer to save the overhead of a call to the smart pointer constructor upon
invocation of the function. You can still pass smart pointer objects to functions
since each smart pointer has an overloaded pointer operator that returns the
underlying raw pointer. The following example illustrates this:
HRESULT DoRasterOp(IRaster* pRaster); // Function dec: raw pointer
IRasterPtr ipRaster;
HRESULT hr = DoRasterOp(ipRaster);
// Pass in smart pointer
Return ArcObjects from functions
This tip builds on the previous one. In this case, raw pointers are used in the
function declaration, and a double indirection is used for the object that will be
returned. This allows you to alter what the pointer you are passed points to.
Next, initialize a smart pointer object with the value you want to return and
assign it to the pointer you were passed.
HRESULT GetTinWorkspace(char* path, ITinWorkspace** ppTinWorkspace)
{
if (!ppTinWorkspace)
return E_POINTER;
HRESULT hr = S_OK;
IWorkspaceFactoryPtr ipWorkspaceFactory(CLSID_TinWorkspaceFactory);
228 • ArcGIS Engine Developer Guide