TStreamer class
To perform schema evolution, old and new classes should be compared;
Class TStreamer keeps information how class was written. Based on this,
TBuffer::DoIt method allows or disallows to Streamer to read current part of TBuffer.
- One instance of TStreamer related to one instance of Tclass;
- It keeps old class check sum. Comparing this with current class, TStreamer makes decision was class modified or not;
- It keeps information about all “atomic” operations of old class;
- Above information is produced by exactly the same code by which automatic Streamer was generated;
- Hash list of TStreamer’s belongs to TFile and saved during Close()
- When TFile is opened this information restored and acceptable to TBuffer