why table and tablebuilder in leveldb use struct rep?

Recently I read the source code of leveldb, but I am confuse about the rep struct in the table and table_builder source.

since we can directly store the member variable directly in the class Table and class TableBuilder.

But why the author make a struct Rep, and store the member variable in the struct Rep.

I can come up whit one reason, because the table and table_builder will expose to the user, so we want to hidden the implementation. Is it right? or there is some other idea that I miss, or is it some design pattern?

Thank you