How to use pybind11::smart_holder with a custom smart pointer

[meghprkh]

Hi, we have a custom shared_ptr that pretty much mimics the API of a std::shared_ptr but is special in other ways

Is there a way to used the smart_holder with a custom shared smart pointer type?

[yangrudan]

The documented custom-holder mechanism is separate from py::smart_holder's built-in conversions. Having a std::shared_ptr-like interface doesn't by itself add your pointer type to smart_holder.

If you want to keep your pointer as the holder, the supported pattern is:

// At global scope, after declaring MySharedPtr and before the bindings:
PYBIND11_DECLARE_HOLDER_TYPE(T, MySharedPtr<T>);

// Inside the module bindings:
py::class_<Widget, MySharedPtr<Widget>>(m, "Widget");

This uses MySharedPtr<Widget> as the holder; it does not extend py::smart_holder. The custom smart pointers documentation covers the expected .get() interface and holder_helper for other accessors. A bound C++ type and its derived types must use a consistent holder.

I would leave the macro's optional third argument at its default unless your ownership rules satisfy the documented condition for true. Constructing another owning pointer from .get() can otherwise create independent owners of the same object.

What special behavior does your pointer add, and which smart_holder feature do you need? That would help distinguish whether a custom holder is sufficient or whether the binding boundary needs an adapter.