增强对哈希唯一索引的多索引访问

这段代码是从boost多索引"mru"的例子中采用的：

http://www.boost.org/doc/libs/1_46_1/libs/multi_index/example/serialization.cpp

我的代码正在执行类似于 boost：：unordered_map 的操作，但我真的很想从此示例中添加 mru 功能。

我想使这段代码尽可能接近 boost：：unordered_map。对我来说，关键功能是unordered_map的 [] 运算符。

main（） 的最后一行被打破，每行上方作为注释是我的问题。

提前感谢所有回答评论。

#include <algorithm>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/sequenced_index.hpp>
#include <iostream>
using namespace boost::multi_index;
class my_struct {
public:
    my_struct(int in_a, std::string in_b) : a(in_a), b(in_b) {}
    int a;
    std::string b;
    bool operator==(const my_struct &rhs) const
    {
        return (a == rhs.a);
    }
    bool operator!=(const my_struct &rhs) const
    {
            return !(*this == rhs);
    }
    friend std::ostream& operator<<(std::ostream &out, const my_struct&ms);
};
std::ostream& operator<<(std::ostream &out, my_struct &ms)
{
    out << ms.a << " " << ms.b << std::endl;
    return out;
}
inline std::size_t
hash_value(const my_struct &val)
{
    return boost::hash_value(val.a);
}
// tags for multi_index
struct umap {};
template <typename Item>
class mru_list
{
  typedef multi_index_container<
    Item,
    indexed_by<
      sequenced<>,
      hashed_unique<boost::multi_index::tag<umap>, identity<Item> >
    >
  > item_list;
public:
  typedef Item                         item_type;
  typedef typename item_list::iterator iterator;
  mru_list(std::size_t max_num_items_):max_num_items(max_num_items_){}
  void insert(const item_type& item)
  {
    std::pair<iterator,bool> p=il.push_front(item);
    if(!p.second){                     /* duplicate item */
      il.relocate(il.begin(),p.first); /* put in front */
    }
    else if(il.size()>max_num_items){  /* keep the length <= max_num_items */
      il.pop_back();
    }
  }
  iterator begin(){return il.begin();}
  iterator end(){return il.end();}
//private:
  item_list   il;
  std::size_t max_num_items;
};
int main()
{
    mru_list<my_struct> mru(10);
    my_struct one(1, "One");
    mru.insert(one);
    mru.insert(my_struct(2, "Two"));
    mru.insert(my_struct(3, "Three"));
    mru.insert(one);
    std::cout<<"most recently entered terms:"<<std::endl;
    for (mru_list<my_struct>::iterator itr = mru.begin(); itr != mru.end(); ++itr) {
        std::cout << itr->a << std::endl;
    }
    // what is my return type?
    mru.il.get<umap>();
    // Why doesn't this work?
    mru_list<my_struct>::iterator itr = mru.il.get<umap>().find(one);
    // Why doesn't this have a [] operator like boost:unordered_map
    mru.il.get<umap>()[1] = "foobar";
    return 0;
}