Call python function with C++ pointers

Question

I want to pass a callable object from Python to C++ and then call it from C++ with arguments for which I have registered base_. Example:

namespace bpy = boost::python;

bpy::class_<X, X*, boost::noncopyable>("X", bpy::no_init);

bpy::def("f", +[](bpy::object fn) {
  fn(new X());
});

Then from python:

from example import f

def fn(x):
  print "x.g() =", x.g()

f(fn)

Throws:

TypeError: No to_python (by-value) converter found for C++ type: X

I have no issue with callable taking other types of arguments, e.g., int, float or some other types I have registered, but this one is failing and I don't understand why: why is a by-value conversion required when I'm passing a X* and I've specified that the held type for X is X*.

Any suggestion on how to fix this?

Full example on Coliru: http://coliru.stacked-crooked.com/a/b3492d2e846d705c

doqtor · Accepted Answer · 2020-06-16 15:40:46Z

1

According to Argument Handling in Calling Python Functions and Methods:

Arguments are converted to Python according to their type. By default, the arguments a1...aN are copied into new Python objects, but this behavior can be overridden by the use of ptr() and ref():

class X : boost::noncopyable { ... };

void apply(PyObject* callable, X& x) { // Invoke callable, passing a Python object which holds a reference to x
boost::python::call(callable, boost::ref(x)); }

It seems that even though the pointer type is specified the implementation takes the base non-pointer type. But in this case either boost::ref or bpy::ptr can be used to fix the problem anyway.

Change:

bpy::def("f", +[](bpy::object fn) {
  fn(new X());
});

into:

bpy::def("f", +[](bpy::object fn) {
    return fn(bpy::ptr(new X()));
});

And now it works:

$ python test.py 
x.g() = 0

answered Jun 16, 2020 at 15:40

doqtor

8,4942 gold badges22 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Holt Over a year ago

Thanks, this is the solution I found right after posting the question. Problem is that in my use case, those are templated stuffs, so I would have to apply bpy::ptr (or boost::ref) depending on the argument, but not all pointer/ref arguments should be wrapped, which makes quite a mess in the end... I wonder the reasoning behind this weird behavior since for python functions defined with bpy::def or bpy::class_::def, the argument is correctly wrap automatically, and same when calling get_override from wrappers.

Collectives™ on Stack Overflow

Call python function with C++ pointers

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related