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4/8/2010

Most of the base classes in the DirectShow class library implement DirectShow COM interfaces. These classes produce C++ objects that provide an IUnknowninterface so external components can access the interfaces that the objects support.

The following topics discuss DirectShow base classes:

CBaseObject and CUnknown Classes

Base Classes that Implement Interfaces

Base Class Constructors

Many constructors in the DirectShow base classes take a pointer to an HRESULTvalue.

If the constructor fails, it sets the value to an error code.

Before calling the constructor, set the HRESULTvalue to S_OK.

When the constructor returns, check the value.

A failure code means the constructor failed, possibly leaving the object in an invalid state.

The following code shows how to check the HRESULTfrom a constructor.

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HRESULT hr = S_OK; // Set the HRESULT to S_OK first. CMemAllocator pAlloc = new CMemAllocator(NAME("MyAllocator"), 0, &hr); if (pAlloc == NULL) { return E_OUTOFMEMORY; } else if (FAILED(hr)) { return E_FAIL; }

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HRESULT hr = S_OK;  // Set the HRESULT to S_OK first.
CMemAllocator pAlloc = new CMemAllocator(NAME("MyAllocator"), 0,
&hr);
if (pAlloc == NULL) 
{
	return E_OUTOFMEMORY;
}
else if (FAILED(hr))
{
	return E_FAIL;
}