CDocument::GetPathName() returns empty string for relative path

I have a requirement where i need to change a relative path to an absolute path when saving a file thru "Save As" . In order to do that i need to get the entire path of the file name of the source path.

The CDocument::GetPathName() should return the absolute path name. But it returns an empty string.
This happens only for the first time. Second time onwards the GetPathName() returns the correct absolute path of the source.

When it happens for the first time, the business logic needs to be executed based on the absolute path of the source. But since it returns the empty string i am unable to proceed

Can anyone help me with this regard?

Well do you know the "default" directory where your files are about to be saved?