ROOT.std.shared_ptr in Python

[oxon]

---

ROOT Version: 6.18/00 + Python 3.7.3
Platform: macOS 10.14
Compiler: Apple LLVM version 10.0.1 (clang-1001.0.46.4)

---

I would like to use std::shared_ptr in PyROOT but only the example I found on the web is Slide 19 of this file.

ROOT-DevCERN-20160530.pdf

from ROOT import std, TH1, TH1F
p = std.shared_ptr<TH1>()
if not p : p = TH1F()
p

But unfortunately it does not work on my Mac with the following error.

In [1]: from ROOT import std, TH1, TH1F                                         
In [2]: p = std.shared_ptr<TH1>()                                               
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-65487fb322c1> in <module>
----> 1 p = std.shared_ptr<TH1>()

TypeError: '<' not supported between instances of 'Template' and 'TH1_meta'

So I tried the commands below instead.

In [3]: p = std.shared_ptr(TH1)()                                               
In [4]: p                                                                       
Out[4]: <ROOT.shared_ptr<TH1> object at 0x7fb4e37bb140>
In [5]: p.get()                                                                 
Out[5]: <ROOT.TH1 object at 0x0>

and it looks OK. However, if I assign a TH1F instance to the pointer as demonstrated in the slide, what I get is not a smart pointer of TH1F but just a normal TH1F object.

In [7]: p = TH1F()                                                              
In [8]: p                                                                       
Out[8]: <ROOT.TH1F object at 0x7fb4e9268700>
In [9]: p.get()                                                                 
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-b38e1d35dbaf> in <module>
----> 1 p.get()

AttributeError: 'TH1F' object has no attribute 'get'

Question: How do I initialize the pointer?

I tried to initialize with an expected constructor, but it did not work.

In [10]: p = std.shared_ptr(TH1)(TH1F())                                        
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-10-79bb6ebcfc11> in <module>
----> 1 p = std.shared_ptr(TH1)(TH1F())

TypeError: none of the 4 overloaded methods succeeded. Full details:
  shared_ptr<TH1>::shared_ptr<TH1>() =>
    takes at most 0 arguments (1 given)
  shared_ptr<TH1>::shared_ptr<TH1>(const shared_ptr<TH1>& __r) =>
    could not convert argument 1
  shared_ptr<TH1>::shared_ptr<TH1>(shared_ptr<TH1>&& __r) =>
    could not convert argument 1 (this method can not (yet) be called)
  shared_ptr<TH1>::shared_ptr<TH1>(nullptr_t) =>
    could not convert argument 1 (void/unknown arguments can't be set)

std::shared_ptr<TH1>::reset dit not work either.

In [11]: p = std.shared_ptr(TH1)()                                              
In [12]: p.reset(TH1F())                                                        
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-76fb6d96ec93> in <module>
----> 1 p.reset(TH1F())

TypeError: void shared_ptr<TH1>::reset() =>
    takes at most 0 arguments (1 given)

[wlav]

Akira,

not sure what the authors of that document were up to, but yes, that first line is an error (it parses as two inequality comparisons) and the assignment is a reference one, which is why they finally print the object, not the shared pointer.

There is an __assign__, but just like __init__ for the constructor, that won’t work b/c those are templated with the Python name divorced from the C++ name. I’d figure the reset could be instantiated, but I guess that fails, too, without a pre-existing template instantiation, b/c the non-templated reset will usurp them.

Not that either of those would have worked in the end, b/c the call TH1F() would have created a TH1F object, then promptly deleted it after the call was finished, leaving the shared_ptr with a dangling pointer.

The canonical way of creating a shared_ptr, in both C++ and Python, however, is the use of std::make_shared. Unfortunately, support for templated free functions is almost non-existent in PyROOT, and neither does it handle the move argument list.

What is left? Not much: you’re stuck with creating the shared_ptrs in C++ using gInterpreter.ProcessLine, after which they’ll be usable (unless you are on MS Windows due to limitations in ROOT/meta).

Aside, all this, however, incl. patches to ROOT/meta for MS Windows, is resolved in cppyy.

[oxon]

Thank you. Yes, ProcessLine works as expected. I will use if for the moment.