Lambda constructor for nb::class_ using nb::init<>.

[arashbm]

With pybind11, it is looks acceptible to use a lambda function with pybind11:init, but I can't get this to work with nanobind::init. Minimal example:

#include <nanobind/nanobind.h>

struct test_struct { int some_date; };

NB_MODULE(nanobind_example_ext, m) {
    nanobind::class_<test_struct>(m, "test_struct")
        .def(nanobind::init([]() -> test_struct { return {.some_date=1}; }));
}

Fails to compile:

/src/nanobind_example_ext.cpp:7:30: error: no matching function for call to ‘init(nanobind_init_nanobind_example_ext(nanobind::module_&)::<lambda()>)’                                

/tmp/pip-build-env-nn1e6js7/overlay/lib/python3.9/site-packages/nanobind/include/nanobind/nb_class.h:428:61: note: candidate: ‘template<class ... Args> nanobind::detail::init<Args ...> nanobind::init()’
428 | template <typename... Args> NB_INLINE detail::init<Args...> init() { return { }; }
    |                                               ^~~~
/tmp/pip-build-env-nn1e6js7/overlay/lib/python3.9/site-packages/nanobind/include/nanobind/nb_class.h:428:61: note:   template argument deduction/substitution failed:
/tmp/pip-req-build-vn02spam/src/nanobind_example_ext.cpp:7:30: note:   candidate expects 0 arguments, 1 provided                                                                       

Is this a removed functionality, or am I making some silly mistake?

[wjakob]

Good catch. This is an area where the syntax differs from pybind11, which wasn't documented.

Please see the README.md file at https://github.com/wjakob/nanobind, specifically the section on "Custom constructors"

[arashbm]

Thanks for the commit. How does this work with the new implicit conversion system? I guess nanobind::is_implicit() plays a role?

[wjakob]

You can define implicit conversion-capable constructors using nb::init_implicit instead of nb::implicit. It's also mentioned in README.md.