// define a function with 3 parameters
Double_t fitf(Double_t *x,Double_t *par)
{
  Double_t arg = 0;
  if (par[2] != 0) arg = (x[0] - par[1])/par[2];
  Double_t fitval = par[0]*TMath::Exp(-0.5*arg*arg);
  return fitval;
}


void userfunctexample() {

// Create a TF1 object using the function defined above. The last
//  parameter specify the number of parameters for the function.

  TF1 * func = new TF1("fit",fitf,-3,3,3);
  // set the parameters to the mean and RMS of the histogram
  func->SetParameters(500,0.,0.5);
  
  // give the parameters meaningful names
  func->SetParNames ("Constant","Mean_value","Sigma");

  // call TH1::Fit with the name of the TF1 object 
  func->Draw();
}