Passing by value

[JendaPlhak]

Assume we have following c++ code -

#include <pybind11/pybind11.h>
#include <iostream>


struct Foo {
    std::string a;
};

void show(Foo f) {
    std::cout << f.a << std::endl;
}

namespace py = pybind11;

PYBIND11_PLUGIN(example) {
    py::module m("example", "pybind11 example plugin");

    m.def("show", &show, "Prints a");
    py::class_<Foo>(m, "Foo")
    .def_readwrite("a",    &Foo::a);

    return m.ptr();
}

And very simple python script where we create Bird structure and want to pass it by value to function show -

import sys
sys.path.append(".")
import example

b = example.Foo
b.a = "Hello"
example.show(b)

This, to me quite surprisingly, causes following error:

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    example.show(f)
TypeError: Incompatible function arguments. The following argument types are supported:
    1. (Foo) -> NoneType

Why is that so? Am I doing it wrong or passing by value is just not possible? I didn't really found anything about this in the docs :-/

[JendaPlhak]

It would be quite alright if I were to pass the argument by reference, but I was unable to accomplish that either.

[wjakob]

Huh? There is nothing called Bird in your example code. Do you mean Foo? Aren't you missing the parentheses, as in Foo()?

[JendaPlhak]

Sorry, already edited. I don't think that it is a problem of parenthesis. Python is quite happy about

f = example.Foo
f.a = "arfaf"
print(f.a)```

So I suppose the object f as such is fine.

[wjakob]

Please just try with parentheses...

[JendaPlhak]

You mean to do - f = example.Foo() ? That doesn't work since I didn't provide constructor for structure Foo. Is that the problem?

[wjakob]

exactly.. please look at the documentation examples before posting here -- my resources for these kinds of support tickets are very limited.

[JendaPlhak]

I see. Ok, sorry to bother, I will try harder next time.