[libcamera-devel] [RFC v3 1/5] HACK: Camera public destructor

[Tomi Valkeinen]

On 09/12/2021 12:05, Kieran Bingham wrote:
> Quoting Tomi Valkeinen (2021-12-09 09:29:02)
>> pybind11 needs a public destructor for Camera to be able to manage the
>> shared_ptr. It's not clear why this is needed, and can it be fixed in
>> pybind11.
>>
> 
> :-( The 'HACK' here seems to be the only thing that would count as a
> blocker to integration.
> 
> I wonder if this is really a HACK or if we just need to explain that the
> destructor needs to be public to support the python bindings, but I
> myself don't understand the reasoning itself.
> 
> Is the python code the only instance where we create a shared_ptr for
> the Camera? If that's true - then perhaps this patch should actually be
> squashed with the one that returns the Camera as a shared_ptr...

This is not related to the patch 2.

pybind11 needs a holder type that is used to manage the references to 
the C++ instances. By default it's unique_ptr, but can be defined. We 
use shared_ptr for many classes 
(https://pybind11.readthedocs.io/en/stable/advanced/smart_ptrs.html?highlight=shared_ptr#std-shared-ptr).

I think normally it makes sense that the destructor must be public: if 
pybind11 wrapper code can create the instances, it can also destruct 
them. Here we don't create Cameras, nor destruct them, but the related 
code is still created, causing the build error.

I believe it should be possible to fix this in pybind11, by making the 
template code to detect these kind of classes and skip the destructor 
code. Maybe. Somehow. I have no idea how =).

  Tomi