我试图弄清楚如何支持const wchar_t *作为boost python中公开的函数的返回类型。我的提升版本是1.52,我正在使用python 2.7,如果这有任何区别。
不知怎的,我无法接受我的转换功能。我已经在互联网上看到了这个问题的解决方案片段,但没有任何实际可行或说明如何做到正确。
这是我琐碎的非工作范例:
#include <string>
#include <boost/python.hpp>
using namespace boost::python;
struct wchar_t_to_python_str
{
static PyObject* convert(const wchar_t* )
{
std::string s = "I'm more interested in the function signature than how to do wide char to non-wide char conversion";
return boost::python::incref(boost::python::object(s).ptr());
}
};
void init_module()
{
to_python_converter<const wchar_t*, wchar_t_to_python_str>();
}
const wchar_t* testWchar() {
return L"Hello World";
}
const char* testChar() {
return "Hello World";
}
BOOST_PYTHON_MODULE(test)
{
// This works nicely, const char* is supported
def("testChar", testChar);
// This doesn't work, fails with this error
// 'awBoost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning<T>'
// def("testWchar", testWchar);
// Throwing in a return value policy fires a compile time assert make_instance_impl
// BOOST_MPL_ASSERT((mpl::or_<is_class<T>, is_union<T> >));
// It seems like it gets confused by wchar_t not being a class, but it's hard to know
def("testWchar", testWchar, return_value_policy<reference_existing_object>());
}
答案 0 :(得分:2)
这个问题有几个因素:
to_python_converter
启用的转换函数是运行时转换。 wchar*
提供builtin conversion支持。它可能被忽略了,因为在更新Boost.Python以支持Python 3时添加了std::wstring
支持。缺少内置转换支持会导致内部类在编译期间需要适当的CallPolicy。所提供的ResultConverterGenerator
模型都不是此转换的候选模型,因为它们主要影响对象所有权/生命周期而不是类型转换。有两种方法可以在这些约束条件下工作:
wchar*
wchar*
的类型包裹返回std::wstring
的函数。以下是展示这两种方法的完整示例:
#include <string>
#include <boost/function_types/parameter_types.hpp>
#include <boost/python.hpp>
/// @brief ResultConverterGenerator used to transform wchar_t to PyObject.
struct wchar_result_converter
{
template <class T> struct apply
{
struct type
{
/// @brief Convert wchar_t to PyObject.
PyObject* operator()(const wchar_t* str) const
{
// Using the Python/C API may be slighly cleaner.
return PyUnicode_FromWideChar(str, wcslen(str));
// Alternatively, Boost.Python's object type can be used. While
// Boost.Python does not know how to convert wchar_t to an object,
// it does know how to convert std::wstring, so construct
// a temporary to help in the conversion.
// return boost::python::incref(
// boost::python::object(std::wstring(str)).ptr());
}
/// @brief Used for documentation.
const PyTypeObject* get_pytype() const { return 0; }
}; // struct type
}; // struct apply
};
/// @brief Modify the return type of a function using supplied CallPolicies.
template <typename ReturnType, typename Fn, typename Policy>
boost::python::object return_as(Fn fn, const Policy& policy)
{
// Build MPL type representing signature of function, injecting the
// explicitly provided return type.
typedef typename boost::mpl::push_front<
typename boost::function_types::parameter_types<Fn>::type,
ReturnType
>::type signature_type;
return boost::python::make_function(fn, policy, signature_type());
}
/// @brief Modify the return type of a function using default_call_policies.
template <typename ReturnType, typename Fn>
boost::python::object return_as(Fn fn)
{
return return_as<ReturnType>(fn, boost::python::default_call_policies());
}
// Test functions.
const char* testChar() { return "Hello World"; }
const wchar_t* testWchar() { return L"Hello World"; }
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
// Expose non-wide test char support.
python::def("testChar", testChar);
// Expose wide char support by:
// - providing a policy to convert the return value.
// - manipulating the return type.
python::def("testWchar1", &testWchar,
python::return_value_policy<wchar_result_converter>());
python::def("testWchar2", return_as<std::wstring>(&testWchar));
}
及其用法:
>>> import example
>>> a = example.testChar()
>>> print a
Hello World
>>> print type(a)
<type 'str'>
>>> b = example.testWchar1()
>>> print b
Hello World
>>> print type(b)
<type 'unicode'>
>>> c = example.testWchar2()
>>> print c
Hello World
>>> print type(c)
<type 'unicode'>