如何使用带有 Boost.Python 的 std::map 或 std::vector 参数的构造函数包装 C++ class？

Question

免责声明：是的，我知道 boost::python::map_indexing_suite。

任务：我有一个 C++ class，我想用 Boost.Python 包装它。它的构造函数接受一个 std::map 参数。这是 C++ header:

// myclass.hh
typedef std::map<int, float> mymap_t;

class MyClass {
  public:
  explicit MyClass(const mymap_t& m);
  // ...
};
// ...

这是 Boost.Python 包装器（仅重要部分）：

// myclasswrapper.cc
#include "mymap.hh"
#include "boost/python.hpp"
#include "boost/python/suite/indexing/map_indexing_suite.hpp"

namespace bpy = boost::python;

// wrapping mymap_t
bpy::class_<mymap_t>("MyMap")
     .def(bpy::map_indexing_suite<mymap_t>())
    ;

// wrapping MyClass
bpy::class_<MyClass>("MyClass", "My example class",
  bpy::init<mymap_t>()   // ??? what to put here?
)
  // .def(...method wrappers...)
;

编译通过。但是，我无法从 Python 端创建映射的 MyClass object，因为我不知道将什么作为参数传递给构造函数。字典不会自动转换为 std::map-s：

# python test
myclass = MyClass({1:3.14, 5:42.03})

口译员抱怨（理所当然）：

Boost.Python.ArgumentError: Python argument types in
    MyClass.__init__(MyClass, dict)
did not match C++ signature:
    __init__(_object*, std::__1::map<int, float, ...

和Python端的MyMap也不能用字典初始化。

用谷歌搜索了一天中最好的部分，我只能找到 "normal" 方法的示例，这些方法采用 std::map 参数映射到 .def(...)。在 .def(...) 中，您不必明确指定映射方法的参数，它们会被神奇地发现。对于构造函数，您必须使用 boost::python::init<...>()，或者至少这是我从文档中了解到的。

问题:

我可以向 MyMap 包装器添加一些内容以帮助 map_indexing_suite 从 Python 字典转换吗？
我可以在 MyClass 包装器的 boost::python::init<...> 中使用不同的模板参数吗？
还有其他想法吗...？

注意：我也看到了this accepted answer at SO，然后我向下滚动并阅读了@YvesgereY的评论：

"For the record, map_indexing_suite solution doesn't work, since no implicit "dict->std::map" from_python converter will be applied."

然后我失去了信心:-)

Answer 1

我找到了一个很好的解决方案：添加了一个可以将 Python 字典转换为 std::map 的模板。该逻辑基于 this extremely useful primer, with slight modifications mostly obtained from this source file 和一些附加评论。

下面是模板定义：

// dict2map.hh
#include "boost/python.hpp"
namespace bpy = boost::python;

/// This template encapsulates the conversion machinery.
template<typename key_t, typename val_t>
struct Dict2Map {

    /// The type of the map we convert the Python dict into
    typedef std::map<key_t, val_t> map_t;

    /// constructor
    /// registers the converter with the Boost.Python runtime
    Dict2Map() {
        bpy::converter::registry::push_back(
            &convertible,
            &construct,
            bpy::type_id<map_t>()
#ifdef BOOST_PYTHON_SUPPORTS_PY_SIGNATURES
            , &bpy::converter::wrap_pytype<&PyDict_Type>::get_pytype
#endif
        );
    }

    /// Check if conversion is possible
    static void* convertible(PyObject* objptr) {
        return PyDict_Check(objptr)? objptr: nullptr;
    }

    /// Perform the conversion
    static void construct(
        PyObject* objptr,
        bpy::converter::rvalue_from_python_stage1_data* data
    ) {
        // convert the PyObject pointed to by `objptr` to a bpy::dict
        bpy::handle<> objhandle{ bpy::borrowed(objptr) };   // "smart ptr"
        bpy::dict d{ objhandle };

        // get a pointer to memory into which we construct the map
        // this is provided by the Python runtime
        void* storage = 
            reinterpret_cast<
                bpy::converter::rvalue_from_python_storage<map_t>*
            >(data)->storage.bytes;

        // placement-new allocate the result
        new(storage) map_t{};

        // iterate over the dictionary `d`, fill up the map `m`
        map_t& m{ *(static_cast<map_t *>(storage)) };
        bpy::list keys{ d.keys() };
        int keycount{ static_cast<int>(bpy::len(keys)) };
        for (int i = 0; i < keycount; ++i) {
            // get the key
            bpy::object keyobj{ keys[i] };
            bpy::extract<key_t> keyproxy{ keyobj };
            if (! keyproxy.check()) {
                PyErr_SetString(PyExc_KeyError, "Bad key type");
                bpy::throw_error_already_set();
            }
            key_t key = keyproxy();

            // get the corresponding value
            bpy::object valobj{ d[keyobj] };
            bpy::extract<val_t> valproxy{ valobj };
            if (! valproxy.check()) {
                PyErr_SetString(PyExc_ValueError, "Bad value type");
                bpy::throw_error_already_set();
            }
            val_t val = valproxy();
            m[key] = val;
        }

        // remember the location for later
        data->convertible = storage;
    }
};

为了使用它，您必须创建一个 Dict2Map 实例以便调用其构造函数。一种可能的方法是在定义 Python 包装器的源文件中创建静态 Dict2Map<key_t, val_t> 变量。使用我的示例：

// myclasswrapper.cc
#include "mymap.hh"
#include "dict2map.hh"

// register the converter at runtime
static Dict2Map<char, double> reg{};

#include "boost/python.hpp" // not really necessary
namespace bpy = boost::python;

// wrapping MyClass
bpy::class_<MyClass>("MyClass", "My example class",
  bpy::init<mymap_t>()
)
  // .def(...method wrappers...)
;

现在可以像这样在 Python 端创建 MyClass 对象：

myclass = MyClass({"foo":1, "bar":2})

编辑：Python 列表可以以类似的方式转换为 C++ std::vector-s。这是相应的模板：

template<typename elem_t>
struct List2Vec {

    /// The type of the vector we convert the Python list into
    typedef std::vector<elem_t> vec_t;

    /// constructor
    /// registers the converter
    List2Vec() {
        bpy::converter::registry::push_back(
            &convertible,
            &construct,
            bpy::type_id<vec_t>()
#ifdef BOOST_PYTHON_SUPPORTS_PY_SIGNATURES
            , &bpy::converter::wrap_pytype<&PyList_Type>::get_pytype
#endif
        );
    }

    /// Check if conversion is possible
    static void* convertible(PyObject* objptr) {
        return PyList_Check(objptr)? objptr: nullptr;
    }

    /// Perform the conversion
    static void construct(
        PyObject* objptr,
        bpy::converter::rvalue_from_python_stage1_data* data
    ) {
        // convert the PyObject pointed to by `objptr` to a bpy::list
        bpy::handle<> objhandle{ bpy::borrowed(objptr) };   // "smart ptr"
        bpy::list lst{ objhandle };

        // get a pointer to memory into which we construct the vector
        // this is provided by the Python side somehow
        void* storage = 
            reinterpret_cast<
                bpy::converter::rvalue_from_python_storage<vec_t>*
            >(data)->storage.bytes;

        // placement-new allocate the result
        new(storage) vec_t{};

        // iterate over the list `lst`, fill up the vector `vec`
        int elemcount{ static_cast<int>(bpy::len(lst)) };
        vec_t& vec{ *(static_cast<vec_t *>(storage)) };
        for (int i = 0; i < elemcount; ++i) {
            // get the element
            bpy::object elemobj{ lst[i] };
            bpy::extract<elem_t> elemproxy{ elemobj };
            if (! elemproxy.check()) {
                PyErr_SetString(PyExc_ValueError, "Bad element type");
                bpy::throw_error_already_set();
            }
            elem_t elem = elemproxy();
            vec.push_back(elem);
        }

        // remember the location for later
        data->convertible = storage;
    }
};

如何使用带有 Boost.Python 的 std::map 或 std::vector 参数的构造函数包装 C++ class？

How to wrap a C++ class with a constructor that takes a std::map or std::vector argument with Boost.Python?

python

boost

stdmap

stdvector

c++11