c++ - 在 boost python 中将 const wchar_t* 转换为 python 字符串

Question

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 boost 版本是 1.52，我正在使用 python 2.7，如果有任何不同的话。

不知怎么的，我无法让它接受我的转换函数。我在 Internet 上看到了解决此问题的片段，但没有任何实际可行的方法或说明如何正确执行此操作。

这是我简单的非工作示例:

#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>());
}

Answer 1

这个问题有几个因素:

• 使用 to_python_converter 启用的转换功能是运行时转换。
• 未提供 Boost.Python builtin conversion支持 wchar*。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。缺少内置转换支持导致内部类需要适当的 CallPolicy。在编译期间。均未提供 ResultConverterGenerator模型是这种转换的候选对象，因为它们主要影响对象所有权/生命周期而不是类型转换。

在这些限制条件下有两种方法:

• 创建自定义策略类型来处理满足 ResultConverterGenerator Concept 的 wchar* .
• 用返回 std::wstring 的类型包装返回 wchar* 的函数。

下面是一个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'>