#include "TCanvas.h" 
#include "TPad.h" 
#include "TH1.h" 
#include "TF1.h" 
#include "TVirtualFitter.h" 
#include "TMath.h" 
TH1 *hist;   //<===============

Double_t fcn_gaussian_area(Double_t *x, Double_t *par ) { 
	
	Double_t  area      = par[0]; 
	Double_t  mean      = par[1]; 
	Double_t  sigma     = par[2]; 
	
	// Find Bin Width 
	//TVirtualFitter *fitter = TVirtualFitter::GetFitter();  //the current fitter 
	//TH1 *hist = (TH1*)fitter->GetObjectFit(); //the histogram being fitted 
	Double_t  bin_width = hist->GetBinWidth(1); 
	
	Double_t y = (x[0] - mean)/sigma; 
	
	const Double_t C = 1.0 / TMath::Sqrt( TMath::TwoPi() ); 
	Double_t amp = C * bin_width * area / sigma; 
	Double_t fitval = amp * TMath::Exp( -y*y/2.0 );
	return fitval; 
	
} 

void Fit( TH1 *h ) { 
	
	// Generate unique name (necessary?) 
	hist=h;  //<=======================
	TString name = h->GetName(); 
	name += "Gaussian"; 
	TF1* gaussian = new TF1( name, fcn_gaussian_area, -5.0, 5.0, 3 ); 
	gaussian->SetParameters( 100, 0.0, 1.0 ); 
	h->Fit(name, "", "", -5.0, 5.0 ); 
	h->DrawCopy();  // Works! 
} 

void Fits() { 
	
	//gROOT->Reset(); //<================
	gDirectory->Delete("h1"); 
	gDirectory->Delete("h2"); 
	
	TH1F * h1 = new TH1F( "h1", "h1", 100, -5.0, 5.0 ); 
	TH1F * h2 = new TH1F( "h2", "h2",  50, -5.0, 5.0 ); 
	
	h1->FillRandom("gaus", 1000 ); 
	h2->FillRandom("gaus", 10000 ); 
	
	TCanvas* myCanvas = new TCanvas( "c1","c1", 20, 20, 400, 200 ); 
	myCanvas->Divide( 2, 1 ); 
	
	myCanvas->cd(1); 
	Fit( h1 ); 
	
	myCanvas->cd(2); 
	Fit( h2 ); 
	
}