#include "TF1.h"
TF1* getf1 ( Double_t cst1 ) 
{ 
   TF1 *f1 = new TF1 ("f1name","[0]*cos(TMath::Pi() *x)", 0., 10.) ;  
   // !!! Here TMath::Pi() OR TMath::DegToRad() WON'T BE TAKEN INTO ACCOUNT !!! 
   f1->SetParameter(0,cst1) ; 
   return (f1) ; 
} 

TF1* mytest ( Double_t cst2 ) 
{ 
   TF1 *f1 = getf1 (2.) ; 
   TF1 *f2 = new TF1 ("f2",f1,0.,10.,0,"TF1") ;
 
   // Here TMath::Pi() OR TMath::DegToRad() CAN BE USED SAFELY 
   //f2->SetParameter(0,cst2) ; 
   return (f2) ; 
} 

void test() { 
   TF1 * f2 = mytest(0);
   f2->Draw();
}