Can I overload init from C++ side in pybind11?

Question

If in C++ a class had an overloaded constructor:

class MyClass {
    std::string name;
public:
    MyClass(){};
    MyClass(std::string name_){ name = name_;}
};

how do I expose this to python? Can I use py::overload_cast and py::init together?

Currently I expose the first one:

PYBIND11_MODULE(ex_module, module_handle){
    module_handle.doc() = "ex class";

    py::class_<MyClass>(module_handle, "PyEx")
        .def(py::init<>())
        ;
}

(Note that I'm using C++20 and python >=3.10)

I want to be able to do:

new_obj = PyEx()
new_obj = PyEx("blah")

Currently, PyEx("blah") isn't throwing any errors, but it's also not really passing blah into the C++ constructor. I check this by adding a print statement in the second constructor.

Edit: simplified the question.

Don't think that error has anything to do with Pybind11. You defined the constructor of Ex to take a single argument keyword_arg... then in the call Ex("blah", keyword_arg="foo") assign it two different values -- first the positional "blah", second the named "foo". — Dan Mašek
– Dan Mašek, Commented Oct 16, 2023 at 17:10
@DanMašek Sure, but then how can I overload the constructor and also have keyword arguments on the python side? Ok maybe I can simplify the question. edit: I simplified the question because I was convoluting two separate issues. — sodiumnitrate
– sodiumnitrate, Commented Oct 16, 2023 at 17:28
The documentation says "To define multiple overloaded constructors, simply declare one after the other using the .def(py::init<...>()) syntax." Have you tried that? — n. m. could be an AI
– n. m. could be an AI, Commented Oct 16, 2023 at 17:49

pptaszni · Accepted Answer · 2023-10-17 07:19:10Z

2

There is no need to use any overload_cast tricks, because exposing the constructor is different than exposing a method where you pass the pointer to member function. You just need to define your single argument constructor after (or before) the default one:

    py::class_<MyClass>(module_handle, "PyEx")
        .def(py::init<>())
        .def(py::init<std::string>())
        ;

answered Oct 17, 2023 at 7:19

pptaszni

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