Deprecate the usage of -> instead of . for objects that are not pointers

[jmcarcell]

An issue in python (wlav/cppyy#312) where a RecursionError is thrown seems to happen because we have an operator-> for collections. Regardless of that, I think it doesn't fit with the rest of podio and should be removed. I have made a few PRs not to use this:

• Use . instead of -> for objects that are not pointers key4hep/k4geo#500
• Use . instead of -> for objects that are not pointers key4hep/k4Reco#39
• Use . instead of -> for EDM4hep objects that are not pointers DD4hep#1480
• Use . instead of -> for objects that are not pointers key4hep/k4FWCore#336

BEGINRELEASENOTES

• Deprecate the generation of an operator-> for collections which made them usable as if they were pointers even when dealing with regular values.
  • Depending on your usage, the removal of this operator might break code that uses a generated datamodel. The recommended fix is to simply switch to using . instead of -> when working with (non-pointer) collections.
  • The original purpose for adding this was the (now abandoned) seamless drop-in replacement for LCIO, which uses collection pointers instead of values.

ENDRELEASENOTES

[m-fila]

Yes! Some time ago we discussed we don't want it in common collection interface and it's mostly for historical reasons

[tmadlener]

Curious that this is only in place for the collections. I thought we also had this for the handles, but I couldn't find it.

[jmcarcell]

The deprecation seems to generate a lot of warnings in python scripts. Reproducer:

import podio
import edm4hep
frame = podio.Frame()
particles = frame.put(edm4hep.MCParticleCollection(), "MCParticleCollection")

and the warnings:

input_line_76:7:90: warning: 'operator->' is deprecated: Use of operator->() is deprecated. Use . instead. [-Wdeprecated-declarations]
      new (ret) (edm4hep::MCParticleCollection*) (((edm4hep::MCParticleCollection*)obj)->operator->());
                                                                                         ^
libedm4hepDict dictionary payload:2494:5: note: 'operator->' has been explicitly marked deprecated here
  [[deprecated("Use of operator->() is deprecated. Use . instead.")]]
    ^
input_line_76:11:53: warning: 'operator->' is deprecated: Use of operator->() is deprecated. Use . instead. [-Wdeprecated-declarations]
      (void)(((edm4hep::MCParticleCollection*)obj)->operator->());
                                                    ^
libedm4hepDict dictionary payload:2494:5: note: 'operator->' has been explicitly marked deprecated here
  [[deprecated("Use of operator->() is deprecated. Use . instead.")]]

[tmadlener]

Does this script still work with #812 in place? If yes, then we could make a tag and merge that one after that.

[tmadlener]

I can't reproduce this with my local stack, where the only obvious major difference is the python version. I can however reproduce this on the nightlies. Building #812 and then EDM4hep on top of the nightlies keeps the script working without the warnings.

AFAIU this the PRs mentioned at the top comment are all "backwards" compatible, i.e. they will keep working with a new podio tag and will work on older podio tags as well. The previous versions however might stop working with #812 merged.

[tmadlener]

@wdconinc @nathanwbrei I don't think you will have any issues with it, but just as a heads up to look out for some potential deprecation warnings when switching to podio@1.4