Using std::unique_ptr<std::string> return type

[romanfomin]

This is simplified example where I add python module with function returning std::unique_ptr<std::string> .

#include <nanobind/nanobind.h>

namespace nb = nanobind;

std::unique_ptr<std::string> Create()
{
    return std::make_unique<std::string>("qwerty");
}

NB_MODULE(mynb, m)
{
    m.def("create", []() { return Create(); });
}

When trying to call this function in python I get error:

>>> import mynb
>>> mynb.create()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Unable to convert function return value to a Python type! The signature was
    create() -> std::unique_ptr<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::default_delete<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >
>>>

How to fix it? I tried to add #include <nanobind/stl/unique_ptr.h> as mentioned in the documentation for unique poiters but this also leds to error.

[hpkfft]

Did you include #include <nanobind/stl/string.h> as well as <nanobind/stl/unique_ptr.h>?

[hpkfft]

Sorry, I didn't think this through before replying, and I don't have time now.
But, if I may ask, why do you want a unique_ptr to a string? A string is already on the heap (if it's not small), so just return a std::string.

[romanfomin]

As I mentioned above this is simplified example. In my project I have more complex return type but with the same error. So I guess solving the problem on this example will help me.

[hpkfft]

It's hard to guess what you really need.
I think you want to include <nanobind/stl/unique_ptr.h> and bind your actual specific class T using something like

nb::class_<T>(m, .......)
   .def(nb::init<>())
    .......

This "regular class binding mechanism" is described in the nanobind docs, with examples.

[wjakob]

The first error message

Conversion of unique_ptr<T> requires that T is handled by nanobind's regular class binding mechanism. However, a type caster was registered to intercept this particular type, which is not allowed.```

already has all the information. You cannot mix unique_ptr and types handled by casting (std::string) in this way. It's not supported and will not be supported in the future.