Extracting a function in shell extension programming - winapi

Question

I want to extract some DropHandler code into a separate function, but have no idea how to do that while working with interface pointers or C++ in general. I want to get just the first item in DragEnter using a separate function.
HRESULT drop_handler::GetFirstItem(IDataObject* p_data_obj, IShellItemArray* items, IShellItem* first_item)
{
HRESULT hr = SHCreateShellItemArrayFromDataObject(p_data_obj, IID_PPV_ARGS(&items));
if (hr != ERROR_SUCCESS)
{
return E_INVALIDARG;
}
DWORD item_count;
items->GetCount(&item_count);
if (item_count != 1)
{
items->Release();
return E_INVALIDARG;
}
hr = items->GetItemAt(0, &first_item);
if (hr != ERROR_SUCCESS)
{
items->Release();
return E_INVALIDARG;
}
return ERROR_SUCCESS;
}
HRESULT drop_handler::DragEnter(IDataObject* p_data_obj, DWORD gtf_key_state, POINTL pt, DWORD* pdw_effect)
{
IShellItemArray* items = nullptr;
IShellItem* dragged_item = nullptr;
HRESULT hr = GetFirstItem(p_data_obj, items, dragged_item);
if (hr != ERROR_SUCCESS)
{
return E_INVALIDARG;
}
//...use dragged_item
This code attempt crashes Explorer big time. I'm not sure what kind of function signature and pointers I should be using to make it work.
Edit: Fixed answer per Anders
HRESULT drop_handler::GetFirstItem(IDataObject* p_data_obj, IShellItemArray*& items, IShellItem*& first_item)
{
HRESULT hr = SHCreateShellItemArrayFromDataObject(p_data_obj, IID_PPV_ARGS(&items));
if (hr != ERROR_SUCCESS)
{
return E_INVALIDARG;
}
DWORD item_count;
hr = items->GetCount(&item_count);
if (hr != ERROR_SUCCESS || item_count != 1)
{
items->Release();
return E_INVALIDARG;
}
hr = items->GetItemAt(0, &first_item);
if (hr != ERROR_SUCCESS)
{
items->Release();
return E_INVALIDARG;
}
return ERROR_SUCCESS;
}
HRESULT drop_handler::DragEnter(IDataObject* p_data_obj, DWORD gtf_key_state, POINTL pt, DWORD* pdw_effect)
{
IShellItemArray* items;
IShellItem* dragged_item;
HRESULT hr = GetFirstItem(p_data_obj, items, dragged_item);
if (hr != ERROR_SUCCESS)
{
return E_INVALIDARG;
}
//...use dragged_item

Answer 1

Your handling of IShellItemArray* and IShellItem* is wrong. GetFirstItem will free IShellItemArray* on failure but on success you will leak it and first_item will never be returned correctly. items and first_item in DragEnter will never be valid.
IShellItemArray* should probably be a local variable in GetFirstItem.
The IShellItem* first_item parameter needs to be IShellItem** first_item or IShellItem*& first_item so that the pointer value is returned correctly to the caller.
You never check the return value of GetCount.
Since you are having problems with pointers you might want to add some asserts to verify that your interface pointers are non-null before using them.