Because the OplogBuffer class can be used to both insert and read from the buffer, code like this is ambiguous:

/** src/mongo/db/repl/oplog_applier.h */

    // Not owned by us.
    OplogBuffer* const _oplogBuffer;

Without prior knowledge, it's not clear if the applier plans to read from this buffer or write to it. Creating an OplogBuffer::Front and OplogBuffer::Back class that only supports either reading to or writing from the OplogBuffer, respectively, would make our code more self-documenting.