I would like to ask how can I use automatic schema evolution when I read and write a class by using TObject::Write() and TObject::Read().
For a test I tried followings.
- Create a file with a class.
// myclass.h
#include "TNamed.h"
class myclass: public TNamed
{
public:
int a;
myclass();
myclass(const char* name);
ClassDef(myclass,1);
};
[code]// myclass.cc
#include "myclass.h"
ClassImp(myclass)
myclass::myclass() {}
myclass::myclass(const char* name){SetName(name);}[/code]
// testclass.cc
#include "myclass.cc"
#include "TFile.h"
int main()
{
static myclass* cuts=new myclass("somename");
cuts->a=4;
TFile f("test.root","recreate");
cuts->Write();
}
$ rootcint -f dict.cc -c myclass.h
$ g++ -Wall root-config --libs --cflags -o testclass testclass.cc dict.cc
$ ./testclass
- Change the class structure
// new myclass.h
#include "TNamed.h"
class myclass: public TNamed
{
public:
int a;
int b; // <== [color=blue]added[/color]
myclass();
myclass(const char* name);
ClassDef(myclass,2); // <== [color=blue]increamented[/color]
};
- Read the file
$ root
root [0] .L myclass.cc+
root [1] TFile f("test.root")
root [2] somename->Dump()
Then it shows an error
Error in TBuffer::CheckByteCount: object of class myclass read too many bytes: 36 instead of 32
Warning in TBuffer::CheckByteCount: myclass::Streamer() not in sync with data on file test.root, fix Streamer()
I guessed one problem is that the file does not have the streamer info of myclass. So I tried adding streamer info of myclass by similar function with TTree::BuildStreamerInfo() and succeeded adding it in the file. I checked with TFile::ShowStreamerInfo().
But it still complains about mismatch of the byte count.
Do I need to write myclass into a TTree to use automatic schema revolution ?