I can use gridfs to do the initial loading with storeFile function. How can I set the _id field for future appending data to the file?

I think I should use GridFileBuilder class, but I didn't find any example in the cxx driver. Can someone help provide an example?

Basically, I want insert data to
dbname.collection_files:
{_id:"unique_string", filename:"my_file_name"...}

After a while, the "my_file_name" would have something append to the end, and I want to append the data based on "unique_string" to the gridfs for the same data.

How can I do it?

Thanks in advance!