如果boost::python::object 引用了一个类型，那么调用它将构造一个具有引用类型的对象：
boost::python::object type = /* Py_TYPE */;
boost::python::object object = type(); // isinstance(object, type) == True
由于 Python 中的几乎所有内容都是对象，因此接受来自 Python 的参数为 boost::python::object 将允许任何类型的对象，即使是那些不是类型的对象。只要对象是可调用的（__call___），那么代码就会成功。
另一方面，如果要保证提供了一个类型，那么一个解决方案是创建一个表示 Python 类型的 C++ 类型，接受它作为参数，并使用自定义转换器来构造 C++ 类型仅当提供 Python 类型时。
以下type_object C++ 类型表示一个 Python 对象，它是一个Py_TYPE。
/// @brief boost::python::object that refers to a type.
struct type_object:
public boost::python::object
{
/// @brief If the object is a type, then refer to it. Otherwise,
/// refer to the instance's type.
explicit
type_object(boost::python::object object):
boost::python::object(object)
{
if (!PyType_Check(object.ptr()))
{
throw std::invalid_argument("type_object requires a Python type");
}
}
};
...
// Only accepts a Python type.
void add_component(type_object type) { ... }
如果提供的PyObject* 是Py_TYPE，则以下自定义转换器将仅构造type_object 实例：
/// @brief Enable automatic conversions to type_object.
struct enable_type_object
{
enable_type_object()
{
boost::python::converter::registry::push_back(
&convertible,
&construct,
boost::python::type_id<type_object>());
}
static void* convertible(PyObject* object)
{
return PyType_Check(object) ? object : NULL;
}
static void construct(
PyObject* object,
boost::python::converter::rvalue_from_python_stage1_data* data)
{
// Obtain a handle to the memory block that the converter has allocated
// for the C++ type.
namespace python = boost::python;
typedef python::converter::rvalue_from_python_storage<type_object>
storage_type;
void* storage = reinterpret_cast<storage_type*>(data)->storage.bytes;
// Construct the type object within the storage. Object is a borrowed
// reference, so create a handle indicting it is borrowed for proper
// reference counting.
python::handle<> handle(python::borrowed(object));
new (storage) type_object(python::object(handle));
// Set convertible to indicate success.
data->convertible = storage;
}
};
...
BOOST_PYTHON_MODULE(...)
{
enable_type_object(); // register type_object converter.
}
这是一个完整的示例demonstrating，它公开了一个需要 Python 类型的函数，然后构造了该类型的一个实例：
#include <iostream>
#include <stdexcept> // std::invalid_argument
#include <boost/python.hpp>
/// @brief boost::python::object that refers to a type.
struct type_object:
public boost::python::object
{
/// @brief If the object is a type, then refer to it. Otherwise,
/// refer to the instance's type.
explicit
type_object(boost::python::object object):
boost::python::object(object)
{
if (!PyType_Check(object.ptr()))
{
throw std::invalid_argument("type_object requires a Python type");
}
}
};
/// @brief Enable automatic conversions to type_object.
struct enable_type_object
{
enable_type_object()
{
boost::python::converter::registry::push_back(
&convertible,
&construct,
boost::python::type_id<type_object>());
}
static void* convertible(PyObject* object)
{
return PyType_Check(object) ? object : NULL;
}
static void construct(
PyObject* object,
boost::python::converter::rvalue_from_python_stage1_data* data)
{
// Obtain a handle to the memory block that the converter has allocated
// for the C++ type.
namespace python = boost::python;
typedef python::converter::rvalue_from_python_storage<type_object>
storage_type;
void* storage = reinterpret_cast<storage_type*>(data)->storage.bytes;
// Construct the type object within the storage. Object is a borrowed
// reference, so create a handle indicting it is borrowed for proper
// reference counting.
python::handle<> handle(python::borrowed(object));
new (storage) type_object(python::object(handle));
// Set convertible to indicate success.
data->convertible = storage;
}
};
// Mock API.
struct GameObject {};
struct CameraComponent
{
CameraComponent()
{
std::cout << "CameraComponent()" << std::endl;
}
};
boost::python::object add_component(GameObject& /* self */, type_object type)
{
auto constructed_type = type();
return constructed_type;
}
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
// Enable receiving type_object as arguments.
enable_type_object();
python::class_<GameObject>("GameObject")
.def("add_component", &add_component);
python::class_<CameraComponent>("CameraComponent");
}
互动使用：
>>> import example
>>> game = example.GameObject()
>>> component = game.add_component(example.CameraComponent)
CameraComponent()
>>> assert(isinstance(component, example.CameraComponent))
>>> try:
... game.add_component(component) # throws Boost.Python.ArgumentError
... assert(False)
... except TypeError:
... assert(True)
...