Below is a complete example ready to compile and link based on your
initial program.
Very likely you forgot the includes !!
Rene Brun
//______________________________________________________________________________
// Simple example with a Tree
// ==========================
//
// This program creates :
// - a ROOT file
// - a TTree with a few branches
// - writes the TTree on the file
//
//*-- Author : Rene Brun 26/06/97
#include "TROOT.h"
#include "TFile.h"
#include "TTree.h"
Int_t Error;
//______________________________________________________________________________
main(int argc, char **argv)
{
TROOT simple("simple","Test of TTree");
TFile hfile("mar.root","RECREATE","ROOT with all info on muons");
typedef struct {Float_t x,y,z;} POINT;
POINT begin_vertex;
POINT end_vertex;
POINT momentum;
Int_t mother;
Float_t p;
Float_t pt;
Float_t track_length;
// this tree contain momenta etc. of several types of muons
TTree *muontree = new TTree("muontree","Data for all muons");
muontree->Branch("Momentum",&momentum,"Px:Py:Pz");
muontree->Branch("Begin_Vertex",&begin_vertex,"Xvert:Yvert:Zvert");
muontree->Branch("EndVertex",&end_vertex,"Xvert2:Yvert2:Zvert2");
muontree->Branch("Tracklength",&track_length,"Tracklength");
muontree->Branch("P",&p,"P");
muontree->Branch("Pt",&pt,"Pt");
muontree->Branch("Mother",&mother,"Mother/I");
muontree->Print();
muontree->Write();
return 0;
}