提振.Python自定义转换器

解决这个问题有两种方法:

• 导出一个辅助函数为Hello.storeFile，接受boost::python::str，从字符串中构造std::vector<unsigned char>，委托给c++ Hello::storeFile成员函数。
编写自定义转换器。虽然转换器不能以每个函数为基础进行注册，但它们的作用域相当好，不会执行任何意外的转换。这种方法通常提供更多的可重用性。

Helper函数

使用辅助函数不会影响任何其他导出函数。因此，python字符串和std::vector<unsigned char>之间的转换只会发生在Hello.storeFile上。

void Hello_storeFile(Hello& self, boost::python::str str)
{
  std::cout << "Hello_storeFile" << std::endl;
  // Obtain a handle to the string.
  const char* begin = PyString_AsString(str.ptr());
  // Delegate to Hello::storeFile().
  self.storeFile(std::vector<unsigned char>(begin, begin + len(str)));
}
...
BOOST_PYTHON_MODULE(hello)
{
  namespace python = boost::python;
  python::class_<Hello>("Hello")
    // This method takes a string as parameter and print it
    .def("printChar", &Hello::printChar)
    // This method takes a vector<unsigned char> parameter
    .def("storeFile", &Hello_storeFile)
    ;
}

转换器注册有三个部分:

• 检查PyObject是否可转换的函数。返回NULL表示PyObject不能使用注册的转换器。
• 从PyObject构造c++类型的构造函数。这个函数只有在converter(PyObject)没有返回NULL时才会被调用。
• 要构造的c++类型。

因此，对于给定的c++类型，如果converter(PyObject)返回一个非NULL的值，那么construct(PyObject)将创建c++类型。c++类型充当进入注册表的键，所以Boost。Python不应该执行非预期的转换。

在这个问题的上下文中，我们想要一个std::vector<unsigned char>的转换器，如果PyObject是PyString, converter(PyObject)将返回非NULL, converter(PyObject)将使用PyObject来创建和填充std::vector<unsigned char>。只有当导出的c++函数具有std::vector<unsigned char>(或const引用)形参并且python提供的实参是字符串时，才会发生此转换。因此，这个自定义转换器不会影响具有std::string参数的导出函数。

下面是一个完整的例子。我选择使转换器通用，以允许从python字符串构造多个类型。有了它的链支持，它应该有和其他Boost一样的感觉。Python类型。

#include <iostream>
#include <list>
#include <string>
#include <vector>
#include <boost/foreach.hpp>
#include <boost/python.hpp>
class Hello
{
public:
  void printChar(const std::string& str)
  {
    std::cout << "printChar: " << str << std::endl;
  }
  void storeFile(const std::vector<unsigned char>& data)
  {
    std::cout << "storeFile: " << data.size() << ": ";
    BOOST_FOREACH(const unsigned char& c, data)
      std::cout << c;
    std::cout << std::endl;
  }
};
/// @brief Type that allows for conversions of python strings to
//         vectors.
struct pystring_converter
{
  /// @note Registers converter from a python interable type to the
  ///       provided type.
  template <typename Container>
  pystring_converter&
  from_python()
  {
    boost::python::converter::registry::push_back(
      &pystring_converter::convertible,
      &pystring_converter::construct<Container>,
      boost::python::type_id<Container>());
    return *this;
  }
  /// @brief Check if PyObject is a string.
  static void* convertible(PyObject* object)
  {
    return PyString_Check(object) ? object : NULL;
  }
  /// @brief Convert PyString to Container.
  ///
  /// Container Concept requirements:
  ///
  ///   * Container::value_type is CopyConstructable from char.
  ///   * Container can be constructed and populated with two iterators.
  ///     I.e. Container(begin, end)
  template <typename Container>
  static void construct(
    PyObject* object,
    boost::python::converter::rvalue_from_python_stage1_data* data)
  {
    namespace python = boost::python;
    // Object is a borrowed reference, so create a handle indicting it is
    // borrowed for proper reference counting.
    python::handle<> handle(python::borrowed(object));
    // Obtain a handle to the memory block that the converter has allocated
    // for the C++ type.
    typedef python::converter::rvalue_from_python_storage<Container>
                                                                 storage_type;
    void* storage = reinterpret_cast<storage_type*>(data)->storage.bytes;
    // Allocate the C++ type into the converter's memory block, and assign
    // its handle to the converter's convertible variable.  The C++
    // container is populated by passing the begin and end iterators of
    // the python object to the container's constructor.
    const char* begin = PyString_AsString(object);
    data->convertible = new (storage) Container(
      begin,                          // begin
      begin + PyString_Size(object)); // end
  }
};
BOOST_PYTHON_MODULE(hello)
{
  namespace python = boost::python;
  // Register PyString conversions.
  pystring_converter()
    .from_python<std::vector<unsigned char> >()
    .from_python<std::list<char> >()
    ;
  python::class_<Hello>("Hello")
    // This method takes a string as parameter and print it
    .def("printChar", &Hello::printChar)
    // This method takes a vector<unsigned char> parameter
    .def("storeFile", &Hello::storeFile)
    ;
}

和示例用法:

>>> from hello import Hello
>>> h = Hello()
>>> h.printChar('abc')
printChar: abc
>>> h.storeFile('def')
storeFile: 3: def
>>> h.storeFile([c for c in 'def'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    Hello.storeFile(Hello, list)
did not match C++ signature:
    storeFile(Hello {lvalue}, std::vector<unsigned char, 
                                          std::allocator<unsigned char> >)

有关自定义转换器和c++容器的更多信息，请考虑阅读此回答