//Illustrates a fit with a member function of a class derived from TF1.
// to use it do:
// root > .L myfit.C+
// root > myfit()
   
#include "TH1.h"
#include "TMath.h"
#include "TF1.h"
   
class myTF1 : public TF1 {
   
public:
   //must define the default and normal constructor
   myTF1(){}
   myTF1(const char*name,Double_t xmin, Double_t xmax, Int_t npar) :
     TF1("*",xmin,xmax,npar) {SetName(name);}
   
   //the function to fit
   Double_t EvalPar(const Double_t *x, const Double_t *par) {
      Double_t arg = 0;
      if (par[2]) arg = (x[0] - par[1])/par[2];
      Double_t fitval = par[0]*TMath::Exp(-0.5*arg*arg);
      return fitval;
   }
     
   ClassDef(myTF1,1)
};

void myfit()
{

   TH1F *hpx = new TH1F("hpx","px distribution",100,-4,4);
   hpx->FillRandom("gaus",25000);
   
// Creates a Root function based on function fitf above
   myTF1 *myfunc = new myTF1("myfunc",-2,2,3);
   
// Sets initial values and parameter names
   myfunc->SetParameters(100,0,1);
   myfunc->SetParNames("Constant","Mean_value","Sigma");

// Fit histogram in range defined by function
   hpx->Fit(myfunc,"r");
}