Question

I am using few CComVariant type variables to store interface pointers.However, sometime I need to pass interface pointer as NULL.In this case when I do this:

CComVariant vAData,vBData;
......
....
CComQIPtr<IBData> pAData = vAData.punkVal; //vAData is {0, VT_I4} when I pass NULL 

CComQIPtr<IBData>pBData = vBData.punkVal;  //vBData is {0, VT_I4} when I pass NULL

The first line fails and throws exception since the vAData.punkVal = 0xffffffff00000000
But the second line passes fine with no error and it has valid vBData.punkVal value(0x0000000000000000).

I am wondering why the two punkVal are different when both are NULL??
Does anybody has any idea why this is happening??
This throws exception only on 64bit machines.

Answer 1

CComVariant calls VariantInit() in constructor and that sets vt to VT_EMPTY, but leaves punkVal uninitialized (doesn’t make it null).

What you try to do is therefore undefined behavior since you try to construct a CComQIPtr passing it an uninitialized pointer.

If you want a CComVariant holding a null IUnknown* you can do this:

CComVariant variant( static_cast<IUnknown*>( 0 ) ); // null IUnknown*, VT_UNKNOWN type

now it’s perfectly legal to construct a CComQIPtr:

CComQIPtr<IWhatever> whatever( variant.punkVal ); //punknVal is null - legal