using boost multi_index_container to preserve insertion order

Question

I initially started out using a std::multimap to store many values with the same key, but then I discovered that it doesn't preserve the insertion order among values with the same key. This answer claims it can be done with boost::multi_index::multi_index_container, but gives no example. Looking through the docs, there are no examples of that usage, and I can't make heads or tails of how you're supposed to use this thing. I have come to expect poor documentation from the lesser-used boost libraries, but this takes the cake. Can anyone point me to a tutorial or example that shows it used the way I want, or perhaps even provide an example themselves?

Answer 1

You could achieve this by using boost::multi_index with two indices: ordered_non_unique(which allows values with the same key) and random_access(which will keep the insertion order).

struct some {
  long key;
  int data;
  int more_data;
  // etc.  
};

typedef multi_index_container<
  some, 
  indexed_by<    
    random_access<>,  // keep insertion order
    ordered_non_unique< member<some, long, &some::key> >
  > 
> some_mic_t;

Answer 2

How about a

map<int, vector<string> >

or

map<int, list<string> >

@Kirill: Good answer. I suspect Boost's random_access can be quite slow, as it will force all strings for all keys to be maintained in a single contiguous structure. Whereas the questioner simply wants order to be preserved within each key's set of mapped values.