Only objects derived from TObject can be written to a file.
I will take a few examples below:
1- Suppose you simply want to use an interactive Root session,
you do not want to define a new class and you only have 2 numbers
(say -89 and 1.2) to write. I would suggest to use a simple histogram
and define xmin and xmax as being your 2 numbers. Later on you can
access these 2 numbers with
Float_t value1 = simple2.GetXmin();
Float_t value2 = simple2.GetXmax();
TH1F simple2("simple2","just 2 numbers xmin and xmax",2,-89,1.2);
TFile f("simple2.root","recreate");
simple2.Write();
In the next session you can access these 2 numbers with
Float_t value1 = simple2.GetXmin();
Float_t value2 = simple2.GetXmax();
2- Suppose you simply want to use an interactive Root session,
you do not want to define a new class and you only have an array
of nvalues to write. I would suggest:
Int_t nvalues = 5;
TH1D simple("simple","just a few numbers",nvalues,0,nvalues);
Double_t values[nvalues] = {1.2,-89,67,122};
simple.SetContent(values);
TFile f("simple.root","recreate");
simple.Write();
In a next session, you can access these numbers like:
Double_t val3 = simple.GetBinContent(3);
3- You have your own compiled class XX derived from TObject.
XX x;
x.SetSomeNumbers(..); // here you initialize your object x
TFile f("simplex.root","recreate");
x.Write("simplex");
4- You have your own compiled class XX derived from TNamed.
XX x("simplex");
x.SetSomeNumbers(..); // here you initialize your object x
TFile f("simplex.root","recreate");
x.Write();
In examples 1,2 and 4, the object written is derived from
the class TNamed. In this case, the Write function creates
automatically a key using the object name.
In case, your object is not derived from TNamed, but simply
from TObject, you must specify the name of the key as a parameter
in the Write function (case 3).
Rene Brun