PySide 通过Shiboken 提供其Qt 绑定。 Shiboken 生成支持其自己的类型转换系统的 Python C API 绑定。这些转换的知识存在于 Shiboken 生成的绑定中，而不是 Python 类型系统中。因此，PySide 知道如何将 QPointF 对象与 C++/Python 相互转换； Python 的类型系统没有。
当对象通过 Boost.Python 公开的函数进行转换时，Boost.Python 将检查其注册表以查找适当的类型转换器。这些转换器为 Boost.Python 提供了有关如何将通过 Boost.Python 公开的类型转换为/从 C++/Python 转换的知识。因此，当 Boost.Python 尝试将 QPointF C++ 类型返回给 Python 时，它会抛出异常，因为该转换尚未在 Boost.Python 中注册。
这里是注释代码：
import myBoostPythonModule
from PySide.QtCore import *
...
x=myBoostPythonModule.X('foo') # Boost.Python knows how to convert C++ X
# to Python X. Python's type system does not.
w=QPointF() # Shiboken knows how to convert C++ QPointF to
# Python QPointF. Python's type system does not.
print(w) # Shiboken knows how to represent C++ QPointF as
# a string.
y=x.getY() # Boost.Python knows how to invoke X::getY(),
# but only Shiboken knows how to convert C++
# QPointF to Python QPointF. Thus, the TypeError
# exception is raised.
Boost.Python 的converters 可以用另一个实现来实现。扩展Shiboken type converter example，下面是使用 Shiboken 的旧类型转换器实现的 Boost.Python 转换器的完整示例。我会使用新的 Shiboken 类型转换器 API，但我不清楚它基于文档是什么。
#include <iostream>
#include <boost/python.hpp>
/// @brief Mockup Complex class from Shiboken documentation.
class Complex
{
public:
Complex(double real, double imaginary)
: real_(real),
imaginary_(imaginary)
{}
double real() const { return real_; }
double imaginary() const { return imaginary_; }
private:
double real_;
double imaginary_;
};
/// @brief Mocked up Shiboken converter.
namespace Shiboken {
template <typename> struct Converter;
template <> struct Converter<Complex>
{
public:
// ...
static inline bool isConvertible(PyObject* pyObj)
{
std::cout << "Shiboken::Converter<Complex>::isConvertible()" << std::endl;
return PyComplex_Check(pyObj);
}
// ...
static inline PyObject* toPython(const Complex& cpx)
{
std::cout << "Shiboken::Converter<Complex>::toPython()" << std::endl;
return PyComplex_FromDoubles(cpx.real(), cpx.imaginary());
}
static inline Complex toCpp(PyObject* pyobj)
{
std::cout << "Shiboken::Converter<Complex>::toCpp()" << std::endl;
double real = PyComplex_RealAsDouble(pyobj);
double imaginary = PyComplex_ImagAsDouble(pyobj);
return Complex(real, imaginary);
}
};
} // namespace Shiboken
/// @brief Type used to convert a complex to Python.
struct complex_converter_to_python
{
static PyObject* convert(const Complex& c)
{
// Delegate to Shiboken.
std::cout << "complex_converter_to_python::convert()" << std::endl;
return Shiboken::Converter<Complex>::toPython(c);
}
};
/// @brief Type that registers a Python Complex type to C++
/// Complex when passing through Boost.Python.
struct complex_converter_from_python
{
/// @note Registers converter from a python complex to C++ complex.
complex_converter_from_python()
{
boost::python::converter::registry::push_back(
&complex_converter_from_python::convertible,
&complex_converter_from_python::construct,
boost::python::type_id<Complex>());
}
/// @brief Check if PyObject is a Complex.
static void* convertible(PyObject* object)
{
// Delegate to Shiboken. Based on the documentation, the
// isConvertible function is gone, so explicit checking may
// be required based on the version of Shiboken.
std::cout << "complex_converter_from_python::convertible()" << std::endl;
return Shiboken::Converter<Complex>::isConvertible(object)
? object
: NULL;
}
/// @brief Convert Python Complex to C++ Complex.
static void construct(
PyObject* object,
boost::python::converter::rvalue_from_python_stage1_data* data)
{
// Obtain a handle to the memory block that the Boost.Python
// converter has allocated for the C++ type.
namespace python = boost::python;
typedef python::converter::rvalue_from_python_storage<Complex>
storage_type;
void* storage = reinterpret_cast<storage_type*>(data)->storage.bytes;
// In-place construct a Complex type via copy-constructor, passing
// in a Complex created from Shiboken.
std::cout << "complex_converter_from_python::construct()" << std::endl;
data->convertible = new (storage) Complex(
Shiboken::Converter<Complex>::toCpp(object));
}
};
/// @brief Factory function used to exercise to-python conversion.
Complex make_complex(double real, double imaginary)
{
return Complex(real, imaginary);
}
/// @brief Printing function used to exercise from-python converison.
void print_complex(const Complex& c)
{
std::cout << "In print_complex: "
<< c.real() << ", "
<< c.imaginary() << std::endl;
}
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
// Register Complex from python converter.
complex_converter_from_python();
// Register Complex to python converter.
python::to_python_converter<
Complex,
complex_converter_to_python>();
python::def("make_complex", &make_complex);
python::def("print_complex", &print_complex);
}
及其用法：
>>> import example
>>> x = example.make_complex(4, 2)
complex_converter_to_python::convert()
Shiboken::Converter<Complex>::toPython()
>>> example.print_complex(x)
complex_converter_from_python::convertible()
Shiboken::Converter<Complex>::isConvertible()
complex_converter_from_python::construct()
Shiboken::Converter<Complex>::toCpp()
In print_complex: 4, 2
另一种方法，虽然不是最优雅的方法，但可以让 Boost.Python 公开的函数使用 boost::python::object 类型，并通过 Python 语句与对象交互。类似于：
boost::python::object X::getY()
{
return boost::python::exec("QPointF()", ...);
}
以上代码将让 Python 实例化一个 QPointF Python 对象，该对象将委托给 Shiboken 的类型系统。由于X::getY() 返回一个泛型对象，当对象句柄从 C++ 转换到 Python 时，Boost.Python 不会尝试执行类型转换。