Getting a TF1 pointer from a ROOT.TF1 class

ROOT
1

Hi,
I have a pretty complex library, which contains a class Func1 which derives from TF1, and which can be instantiated like this:

Func1(const &TF1)

I have built a python wrapper of the library with SWIG, so I can access its classes from python.

Then, I have a python function which cannot be easily re-written in C, and I can use it to get a ROOT.TF1 object from pyROOT:

tf1 = ROOT.TF1(“test”,someCrazyPythonFunction,-10,10,1)

Then, from python, I need to get a Func1 object which uses the ROOT.TF1 class I just created. Of course, doing:

Func1(tf1)

does not work, because tf1 is not a TF1 class, but a ROOT.TF1 class.

Is there a way to get the underlying TF1 C-class from a ROOT.TF1 class?

2

Hi Giacomo,

a question to understand better your setup before actually debugging this particular problem: why did you decide for swig instead of ROOT dictionaries?
This problem could be solved as follows:
Func1 class

#include "TF1.h"
class Func1:public TF1 {
public:
	Func1(TF1& f){};

ClassDef(Func1, 1)
};

Selection xml

<lcgdict>
<class name="Func1" />
</lcgdict>

Dictionary generation and compilation of the library (ROOT6, ~identical for ROOT5)

genreflex Func1.h -s sel.xml --rootmap al.rootmap 
g++ -o libFunc1.so -shared -fPIC `root-config --cflags --libs` Func1_rflx.cpp

The rootmap file is used by ROOT to dynamically load a library when a symbol is missing. Libraries can be of course also loaded manually via gSystem->Load(“libname”).
In Python

>>> import ROOT
>>> f = ROOT.TF1()
>>> func1 = ROOT.Func1(f)
>>> func1
<ROOT.Func1 object at 0x7fe15e5128c0>
3

Thanks for the reply!

Unfortunately the library I am using is very complex, with a lot of external third-party libraries being imported and used, among which boost.

I tried to get a dictionary of everything I need and to import the library in ROOT but, after spending a day inserting #ifndef CINT and similar, I gave up. rootcint cannot parse most of the headers of my library…

Basically I reached the point you reached (having a ROOT.Func1 instance), but then I need to pass that class to other classes of the library, and the problem presents itself again: they want a Func1 class, not a ROOT.Func1 class…

4

Hi,

[quote=“giacomov”]Is there a way to get the underlying TF1 C-class from a ROOT.TF1 class?[/quote]yes, PyROOT and SWIG can talk CObjects with each other. My SWIG is rusty, but the two calls you need from PyROOT are:ROOT.AsCObject( tf1 ) # produces PyCObject from TF1 ROOT.BindObject( cobj_from_swig, ROOT.TF1 ) # produces TF1 from PyCObject
HTH,
Wim

5

Thanks!

Unfortunately as far as I can tell, ROOT.BindObject support only objects which are known to ROOT, and it returns again a ROOT.* object. In my case I would have to generate a dictionary for Func1 (which is easy), load the library (.so) with gSystem.Load(), and then use BindObject, but I will end up again with a ROOT.Func1 class, instead of a Func1 class.

I now realize that this might not be the right place for this, since I guess the real issue here is how to get a Func1 class from a void pointer, in SWIG.

Anyway, if anyone has any more advice, it would be much appreciated.

6

Hi,

yes, of course, that is the point.

You extract the CObject from ROOT, bind it to SWIG, do all the SWIG thingies, re-extract a CObject from SWIG, then bind it back to ROOT to do ROOT thingies. (Of course, if the object address stays the same, rebinding is not necessary: just keep a reference alive to the old python object.)

Not following this … you said in your first post that you had SWIG bindings?

Cheers,
Wim

7

sorry, I didn’t make myself very clear…

This is what I need to do:

tf1 = ROOT.TF1(…)
f1 = mySwigWrapper.Func1(tf1)
oc = mySwigWrapper.OtherClass(f1)

(and then many other classes from mySwigWrapper)

So, I need f1 to be of type Func1, not ROOT.Func1, otherwise I cannot create the instance of OtherClass.

I cannot use rootcint on all classes of the library because it cannot parse them.

I have written a SWIG interface so that I can do this:

tf1 = ROOT.TF1(…)
f1 = mySwigWrapper.Func1(ROOT.AsCObject(tf1))

as you suggested, but now f1 is of type CObject, and I need to “cast it back” to Func1, and I don’t know how to do it…

8

Hi,

okay, that’s up to SWIG then. A cast or typemap, or whatever the things were called (sorry, too long ago). Use the Python C-API “PyCObject_AsVoidPtr” to extract the pointer value on the SWIG side (in the .i).

Cheers,
WIm

9

Thanks!

Yes, I have been playing with that all day (and the night, I just finished and it’s 1am…), but I was missing a small key part, i.e., the (TF1 *) casting in the constructor of Func1.

Just for reference, this is some relevant snipplets:

[code]class Func1: public TF1 {

public:

Func1() : TF1() {};

//This constructor is primarily for the Python wrapper, so that Func1
//can be created starting from an arbitrary ROOT.TF1 object, casted to void*
//by using ROOT.AsCObject and the appropriate SWIG typemap
Func1(const char* name,void* mfcn, Double_t xmin, Double_t xmax, Int_t npar) : 
  TF1(name,(TF1 *)mfcn,xmin,xmax,npar) {};

Func1(const char* name, const char* formula, double xmin = 0, double xmax = 1):
  TF1(name, formula, xmin, xmax) {};

}[/code]

I added this in the interface file (.i):

[code]%typemap(in) void *mfcn {
$1 = PyCObject_AsVoidPtr($input);
}

%rename (Func1_fromVoid) Func1(const char* name, void* mfcn, Double_t xmin, Double_t xmax, Int_t npar);[/code]

And this is how I can use it now:

import ROOT
import mySwigWrapper
tf1 = ROOT.TF1("test","[0]*x+[1]")
myfunc1 = mySwigWrapper.Func1_fromVoid("testFunc1",ROOT.AsCObject(tf1),-10,10,2)
print(myfunc1)
<mySwigWrapper.Func1; proxy of <Swig Object of type 'Func1 *' at 0x7fd5e33d6b40> >

In the Func1 header, note the (TF1 *) casting when initializing the base class TF1: it took me a long time to realize that I had to add that…

Now I can go on and use myfunc1 with all the other classes of the library.

Thanks for the help!