how to test if CComBSTR is empty

How to test if a CComBSTR is an empty string? (with no 'text' value, can be "" or can be null)

my ideas:

1. test if CComBSTR::ByteLength() returns 0
2. test if CComBSTR::GetStreamSize() returns 0
3. test if CComBSTR::m_str is NULL
4. test if CComBSTR::Length() returns 0

which one is correct approach? if none of them is, then what is correct approach?

thanks.

4) Test length 0 it's fast as it's stored