Boost.Python: expose class member which is a pointer

Question

I have a C++ class that would like to expose to Python. (Assuming this class has already been written and couldn't be easily modified). In this class, there is a member which is a pointer, and I would like to expose that member as well. Here is a minimal version of the code.

struct C {
  C(const char* _a) { a = new std::string(_a); }
  ~C() { delete a; }
  std::string *a;
};


BOOST_PYTHON_MODULE(text_detection)
{
  class_<C>("C", init<const char*>())
      .def_readonly("a", &C::a);
}

It compiles okay, except that there is a Python runtime error when I tried to access that field:

>>> c = C("hello")
>>> c.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: No to_python (by-value) converter found for C++ type: std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*

which is understandable. But the question is that is it possible to expose member pointer a through Boost Python at all? And how?

Praetorian · Accepted Answer · 2016-04-12 04:53:39Z

5

Instead of using def_readonly, use add_property with a custom getter. You'll need to wrap the getter in make_function, and since the getter is returning a const& you must also specify a return_value_policy.

std::string const& get_a(C const& c)
{
  return *(c.a);
}

BOOST_PYTHON_MODULE(text_detection)
{
  using namespace boost::python;
  class_<C>("C", init<const char*>())
    .add_property("a", make_function(get_a, return_value_policy<copy_const_reference>()))
    ;
}

Live demo

answered Apr 12, 2016 at 4:53

Praetorian

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