A ROOT Problem:How to use TF1 to do function integration

From: Feng Xiong (fxiong@mit.edu)
Date: Fri Aug 27 1999 - 06:57:11 MEST 

Hi Rooters,

I am a beginner in C++ and I had a problem when I was trying to use ROOT's
class TF1 to do integration. I believe it's very simple by nature.

Basically in my class PRCHe3 there is a member function IntegrandERT() to
calculate cross section(elastic radiative tail) as a function of some
parameter t (it's very complicated in form and has to call several Fortran
subroutines, it also depends on some private members of the class, thus I
can not make it an ordinary function), and I want to integrate over a
certain range of t to get my result. A simple idea is to define a TF1
object for this member function and thus utilize the Integral() member
function of TF1. However the TF1 constructor only takes a pointer to the
function, and I can not define a pointer to the member function(though
it'a all right for ordinary functions, as in the example of stress1 in
test/stress.cxx), like:
       
       Double_t PRCHe3::IntegrandERT(Double_t *x, Double_t *p)
       {
          ...
       }       

       Double_t (*func) (Double_t *, Double_t *);
       func = IntegrandERT;
       TF1 *f1 =  new TF1("elastic", func, -10, 10, 5); 

or segmentation violation occurs -- though it compiles and links, with
a warning like: 

PRCHe3.cxx:178: warning: assuming & on `PRCHe3::IntegrandERT(Double_t *, Double_t *)'
PRCHe3.cxx:178: warning: converting from `Double_t (PRCHe3::*)(Double_t *, 
Double_t *)' to `Double_t (*)(Double_t *, Double_t *)'

Then I tried to define the member function something like

   Double_t PRCHe3::(*IntegrandERT)(Double_t *x, Double_t *p) { ... }

but it did not work.

Can anybody show me how to solve this problem? Many thanks!

Best regards,
Feng

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