[init_implicit]How to enable a tuple to be implicit converted to a class?

[huangweiwu]

I have a class point below, but the init_implicit code comes up with a compile error.
Cannot convert from 'initializer list' to 'point'

nb::class_<point>(m, "point)
    .def(nb::init<float, float>())
    .def(nb::init_implicit<std::tuple<float, float>>())

[wjakob]

Do you have a constructor taking that exact argument? Perhaps you are missing a & or const

[wjakob]

Aha. Yes, that of does not work because this constructor does not exist, and implicit conversion from float to int won't be done. The documentation explains how you can make a custom __init__ style constructor and sign it up for implicit conversions.

[huangweiwu]

Boost python has a way to register default implicit conversion.
Using boost::python::converter::registry to register check function and construct function.
In nanobind, is there any other ways to make default implicit conversion besides init_implicit.

[wjakob]

Not really an officially supported way, but you can do:

nb::detail::implicitly_convertible(
    [](PyTypeObject *dst, PyObject *src, detail::cleanup_list *cleanup) noexcept -> bool {
        /* Return whether or not `src` is implicitly convertible to a Python type `dst`. */
    },
    &typeid(TargetCPPType) /* <-- this is the C++ type associated with the binding */);

This is what nanobind uses to decide whether or not to call a constructor of some class. That constructor must actually exist though.

[huangweiwu]

It seems that the corresponding constructor is indispensable.