Boost.Python and Polymorphic Behaviour with std::shared_ptr

Question

I would like to know how classes A and B below can work polymorphically in python when using std::shared_ptr instead of boost::shared_ptr?

struct A
{
    virtual ~A() {}
};

struct B : A
{
    B() {}
    virtual ~B() {}
};

void f(const std::shared_ptr<A>& ptr)
{}

BOOST_PYTHON_MODULE(test)
{
    class_<A, boost::noncopyable>("A", no_init);

    class_<B, std::shared_ptr<B>, bases<A>>("B")
        .def(init<>());

    def("f", f);
}

I am aware that the the boost::get_pointer method must be defined for std::shared_ptr, so I make sure that the following lines exist before a #include <boost/python.hpp>:

namespace boost {
template<class T> const T* get_pointer(const std::shared_ptr<T>& p)
{
    return p.get();
}

template<class T> T* get_pointer(std::shared_ptr<T>& p)
{
    return p.get();
}
} // namespace boost

Now, in python I try:

>>> from test import *
>>> b = B()
>>> f(b)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
ArgumentError: Python argument types in
    test.f(B)
did not match C++ signature:
    f(std::shared_ptr<A>)

Note: The code above works fine for boost::shared_ptr, but I want to stick with the C++11 types instead.

Thanks.

If anyone else stumbles upon this, @eudoxos has a working solution that below. You don't need to switch to `boost::shared_ptr`. — Michael Koval, May 21 '15 at 15:41

score 0 · Answer 1 · answered Apr 17 '12 at 14:02

0

Boost.Python has special magic built in to recognize a boost::shared_ptr. It does not currently recognize a std::shared_ptr. For now, just frown and use boost::shared_ptr.

answered Apr 17 '12 at 14:02

Matthew Scouten

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Boost.python can use std::shared_ptr via get_pointer, what you write is not true. – eudoxos Apr 18 '12 at 20:52
2

std::shared_ptr is only recently supported. In boost 1.47 it is not there. – mirk Apr 19 '12 at 13:47

score 0 · Answer 2 · edited May 23 '17 at 10:28

0

I made once std::shared_ptr work with boost::python IIRC, using get_pointer as you write. (The relevant comment is here). My guess is that you need to have class_<A,std::shared_ptr<A>,...> since that is what your function takes as argument.

edited May 23 '17 at 10:28

Community

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1

answered Apr 18 '12 at 20:57

eudoxos

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I tried `class_>("A")`, but no luck. It compiles fine, but it fails in python. – A.L. Apr 19 '12 at 13:29
Did you actually post a **real** code?? I am unable to compile that at all without some modifications. Could you, please? – eudoxos Apr 20 '12 at 07:30
I posted on the c++-sig forum [here](http://thread.gmane.org/gmane.comp.python.c++/15759), because it looks like a bug or at least unexpected behavior. Let's see what the boost:;python guys say. – eudoxos Apr 20 '12 at 08:02
3

There is an answer at c++-sig [here](http://thread.gmane.org/gmane.comp.python.c++/15759/focus=15771). To summarize, you have to say `implicitly_convertible,std::shared_ptr>(); `. – eudoxos Apr 26 '12 at 18:00

Boost.Python and Polymorphic Behaviour with std::shared_ptr

2 Answers2