boost.python: Argument types did not match C++ signature

Question

I am having a strange problem when calling a C++ function in python.

I exposed a class from which I want to call a function:

class_<MyClass, std::shared_ptr<MyClass>>("MyClass", init<>())
    // ...
    .def("someFunc", &MyClass::someFunc)
;

I get a std::shared_ptr<MyClass> from a member variable from another class which is exposed via .def_readonly(...)

When I try to call the function, I get the following error:

File "pytest.py", line 27, in test_func
    cu.someFunc("string")
Boost.Python.ArgumentError: Python argument types in
    MyClass.someFunc(MyClass, str)
did not match C++ signature:
    result(MyClass{lvalue}, std::string)

As far as I see, the signatures do match. Does someone see the problem?

Solution

As tracked in this ticket, Boost.Python does not fully support std::shared_ptr.

In short, two easy solutions are to either:

• Use boost::shared_ptr instead of std::shared_ptr.
• Expose the std::shared_ptr member variable with via add_property(), providing a boost::python::return_value_policy with a type of boost::python::return_by_value.

---

While the signatures in the exception look the same, the subtle detail is that the Python MyClass object embeds a std::shared_ptr<MyClass>. Thus, Boost.Python must perform a conversion from an std::shared_ptr<MyClass> to an lvalue MyClass. However, Boost.Python does not currently support custom lvalue conversions. Thus, an ArgumentError exception is thrown.

When exposing member variables with def_readonly("spam", &Factory::spam), it is the equivalent of exposing it via:

add_property("spam", make_getter(&Factory::spam, return_internal_reference()))

Boost.Python has special code when the type being exposed in this manner is a boost::shared_ptr. As it is a read-only property and std::shared_ptr is intended to be copied, it is safe to expose a copy of std::shared_ptr with a return value policy with a type of return_by_value.

Here is a complete example where Factory exposes a Spam object held by std::shared_ptr and an Egg object held by boost::shared_ptr:

#include <iostream>
#include <memory> // std::shared_ptr, std::make_shared
#include <string>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/python.hpp>

/// @brief Mockup Spam type.
struct Spam
{
  ~Spam() { std::cout << "~Spam()" << std::endl; }

  void someFunc(std::string str)
  {
    std::cout << "Spam::someFunc() " << this << " : " << str << std::endl;
  }
};

/// @brief Mockup Egg type.
struct Egg
{
  ~Egg() { std::cout << "~Egg()" << std::endl; }

  void someFunc(std::string str)
  {
    std::cout << "Egg::someFunc() " << this << " : " << str << std::endl;
  }
};

/// @brief Mockup Factory type.
struct Factory
{
  Factory()
    : spam(std::make_shared<Spam>()),
      egg(boost::make_shared<Egg>())
  {
    spam->someFunc("factory");
    egg->someFunc("factory");
  }

  std::shared_ptr<Spam>   spam;
  boost::shared_ptr<Egg>  egg;
};

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;

  // Expose Factory class and its member variables.
  python::class_<Factory>("Factory")
    // std::shared_ptr<Spam>
    .add_property("spam", python::make_getter(&Factory::spam,
        python::return_value_policy<python::return_by_value>()))
    // boost::shared_ptr<Egg>
    .def_readonly("egg", &Factory::egg)
    ;

  // Expose Spam as being held by std::shared_ptr.
  python::class_<Spam, std::shared_ptr<Spam>>("Spam")
    .def("someFunc", &Spam::someFunc)
    ;

  // Expose Egg as being held by boost::shared_ptr.
  python::class_<Egg, boost::shared_ptr<Egg>>("Egg")
    .def("someFunc", &Egg::someFunc)
    ;
}

Interactive Python demonstrating usage and object lifetime:

>>> import example
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8d5dbc9 : factory
>>> factory.spam.someFunc("python")
Spam::someFunc() 0x8d73250 : python
>>> factory.egg.someFunc("python")
Egg::someFunc() 0x8d5dbc9 : python
>>> factory = None
~Egg()
~Spam()
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8d06569 : factory
>>> spam = factory.spam
>>> factory = None
~Egg()
>>> spam.someFunc("python")
Spam::someFunc() 0x8d73250 : python
>>> spam = None
~Spam()
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8ce10f9 : factory
>>> egg = factory.egg
>>> factory = None
~Spam()
>>> egg.someFunc("python")
Egg::someFunc() 0x8ce10f9 : python
>>> egg = None
~Egg()