Just FYI, every time I call Open... in the native file open dialog, it always begins from my local Documents folder. That tells me that they are using one of the Apple standard location file handles in their call. This is a multi-platform method that attempts to become OS independent, where certain kinds of files are stored in a Documents folder, others in App Support folders, etc.
I don't know the names used in C/C++ on OS X, but in Lisp we have a function called (Get-Folder-Path <arg>) where we can specify as argument an OS agnostic path spec, :documents, :appdata, :my-documents, :common-appdata, :common-documents, etc. On some OS some of these may refer to the same location.
- DM