为什么boost::python迭代器跳过第一个元素

当我试图在C++中为Python实现可迭代对象时（使用boost:：Python），我遇到了一个奇怪的问题。Python似乎总是提前取消引用一个元素，所以，结果它跳过了第一个元素，还取消引用了"end"元素。我也不确定我的返回值策略是否选择正确，但如果我用std:：string替换int作为元素类型，它似乎是唯一一个正确工作的策略。迭代器标记是特意选择的——我打算实现可迭代对象来访问只能遍历一次的资源。

C++代码：

#include <Python.h>
#include <boost/python.hpp>
#include <iostream>
#include <iterator>
int nextInstance{0};
class Foo
{
public:
    class iterator : public std::iterator<std::input_iterator_tag, int>
    {
    public:
        iterator() = delete;
        iterator& operator=(const iterator&) = delete;
        iterator(const iterator& other)
        :
            instance_(nextInstance++),
            pos_(other.pos_)
        {
            std::cout << instance_ << " copy ctor " << other.instance_ << " (" << pos_ << ")n";
        }
        explicit iterator(int pos)
        :
            instance_(nextInstance++),
            pos_(pos)
        {
            std::cout << instance_ << " ctor (" << pos_ << ")n";
        }
        bool operator==(iterator& other)
        {
            std::cout << instance_ << " operator== " << other.instance_ << " (" << pos_ << ", " << other.pos_ << ")n";
            return pos_ == other.pos_;
        }
        int& operator*()
        {
            std::cout << instance_ << " operator* (" << pos_ << ")n";
            return pos_;
        }
        iterator operator++(int)
        {
            ++pos_;
            std::cout << instance_ << " operator++ (" << pos_ << ")n";
            return *this;
        }
        ~iterator()
        {
            std::cout << instance_ << " dtorn";
        }
    private:
        const int instance_;
        int       pos_{0};
    };
    iterator begin()
    {
        std::cout << "begin()n";
        return iterator(0);
    }
    iterator end()
    {
        std::cout << "end()n";
        return iterator(3);
    }
};
BOOST_PYTHON_MODULE(pythonIterator)
{
    boost::python::class_<Foo, boost::noncopyable>("Foo", boost::python::init<>())
        .def("__iter__", boost::python::iterator<Foo, boost::python::return_value_policy<boost::python::copy_non_const_reference>>{});
}

Python代码：

#!/usr/bin/python
import pythonIterator
foo = pythonIterator.Foo()
for i in foo:
    print i

输出：

end()
0 ctor (3)
begin()
1 ctor (0)
2 copy ctor 1 (0)
3 copy ctor 0 (3)
1 dtor
0 dtor
4 copy ctor 2 (0)
5 copy ctor 3 (3)
3 dtor
2 dtor
4 operator== 5 (0, 3)
4 operator++ (1)
6 copy ctor 4 (1)
6 operator* (1)
6 dtor
1
4 operator== 5 (1, 3)
4 operator++ (2)
7 copy ctor 4 (2)
7 operator* (2)
7 dtor
2
4 operator== 5 (2, 3)
4 operator++ (3)
8 copy ctor 4 (3)
8 operator* (3)
8 dtor
3
4 operator== 5 (3, 3)
5 dtor
4 dtor