void write_to_file ( ) {
// I s t a n c e o f our hi s togram
TH1F h("my_histogram","My Title;X;# of entries",100,-5,5);

// Let's fill it randomly
h.FillRandom( "gaus" );

// Let's open a TFile
TFile out_file("my_rootfile.root","RECREATE");

// Write the hi s togram in the f i l e
h.Write();

// Close the file
out_file.Close();
}