Write Objects to a File

ROOT
1

Dear experts,
I’m trying to write an object inherited from TObject following instructions here: https://root.cern.ch/root/HowtoWrite.html.
The problem is that I can not read back the values… and I have no idea if the problem is in writing or reading.

The code I used to do the test is:

#include <iostream>
#include <TChain.h>
#include <TFile.h>
#include <TBuffer.h>

class Test1:public TObject{
  public:
  Test1():a(999),b(999){};
  Test1(float _a, float _b):a(_a),b(_b){std::cout << "a=" << a << " b=" << b << std::endl;}
  float a{-999};
  float b{-999};

  void Streamer(TBuffer &bf)
   {
     if (bf.IsReading()) {
        Version_t v = bf.ReadVersion();
        TObject::Streamer(bf);
        bf >> a;
        bf >> b;
     } else {
        bf.WriteVersion(Test1::IsA());
        TObject::Streamer(bf);
        bf << a;
        bf << b;
        std::cout << "Writing: a=" << a << " b=" << b << std::endl;
     }
   }

//   ClassDef(Test1,1)  //The class title
};

int write(float x=-1, float y=1){
  TFile f1("x1.root","recreate");
  Test1 t1(x,y);
  t1.Write("t1");

  return 0;
}

int read(){
  TFile f1("x1.root","read");
  Test1* t1 = (Test1*)f1.Get("t1");
  std::cout << "read a=" << t1->a << " b=" << t1->b << std::endl;

  return 0;
}

int main(){
  write(-4, 9);  
  return read();
}

There is no error/warning message in compilation. But the output of this test code is:

a=-4 b=9
Writing: a=-4 b=9
read a=4.48416e-44 b=0

The last line is expected to be

read a=-4 b=9

Anything wrong in this test code? Thanks!

Dongliang

2

Hello,

If you use ClassDef to generate the streamer it works.
Why do you want to define your own streamer?

G Ganis

3

Thanks for the reply. I tried to make it simple by keeping everything in a single file…
Now I find ClassDef is even simpler. I will go with it.

4

Note that when inheriting from TObject, the ClassDef macro is compulsory.