Question

Suppose I have some data stored in a container of unique_ptrs:

struct MyData {
    int id;  // a unique id for this particular instance
    data some_data; // arbitrary additional data
};

// ...

std::vector<std::unique_ptr<MyData>> my_data_vec;

The ordering of my_data_vec is important. Suppose now I have another vector of IDs of MyDatas:

std::vector<int> my_data_ids;

I now want to rearrange my_data_vec such that the elements are in the sequence specified by my_data_ids. (Don’t forget moving a unique_ptr requires move-semantics with std::move().)

What’s the most algorithmically efficient way to achieve this, and do any of the STL algorithms lend themselves well to achieving this? I can’t see that std::sort would be any help.

Edit: I can use O(n) memory space (not too worried about memory), but the IDs are arbitrary (in my specific case they are actually randomly generated).

Answer 1

1. Create a map that maps ids to their index in my_data_ids.
2. Create a function object that compares std::unique_ptr<MyData> based on their ID’s index in that map.
3. Use std::sort to sort the my_data_vec using that function object.

Here’s a sketch of this:

// Beware, brain-compiled code ahead!
typedef std::vector<int> my_data_ids_type;
typedef std::map<int,my_data_ids_type::size_type> my_data_ids_map_type;

class my_id_comparator : public std::binary_function< bool
                                                    , std::unique_ptr<MyData>
                                                    , std::unique_ptr<MyData> > {
public:
  my_id_comparator(const my_data_ids_map_type& my_data_ids_map)
    : my_data_ids_map_(my_data_ids_map) {}

  bool operator()( const std::unique_ptr<MyData>& lhs
                 , const std::unique_ptr<MyData>& rhs ) const
  {
     my_data_ids_map_type::const_iterator it_lhs = my_data_ids_map_.find(lhs.id);
     my_data_ids_map_type::const_iterator it_rhs = my_data_ids_map_.find(rhs.id);
     if( it_lhs == my_data_ids_map_.end() || it_rhs == my_data_ids_map_.end() )
       throw "dammit!"; // whatever
     return it_lhs->second < it_rhs->second;
  }
private
  my_data_ids_map_type& my_data_ids_map_;
};

//...

my_data_ids_map_type my_data_ids_map;
// ...
// populate my_data_ids_map with the IDs and their indexes from my_data_ids
// ...
std::sort( my_data_vec.begin(), my_data_vec.end(), my_id_comparator(my_data_ids_map) );

If memory is scarce, but time doesn’t matter, you could do away with the map and search the IDs in the my_data_ids vector for each comparison. However, you would have to be really desperate for memory to do that, since two linearly complex operations per comparison are going to be quite expensive.