what is the best way to use the C type uuid_t as a key in a std::map?

Question

Is this an appropriate way to provide unique keys in a map? In other words, is the key being made from the unique value contained in the uuid, or is it being made from the pointer to the uuid_t struct? A side question, is there a more efficient container, when I don't care about the ordering by keys inside the container?

#include <uuid/uuid.h>

int main(int argc, char **argv)
{    
   std::map<uuid_t,int> myMap;         

   uuid_t id1;
   uuid_t id2;

   uuid_generate( (unsigned char *)&id1 );  
   uuid_generate( (unsigned char *)&id2 );

   myMap[id1] = 5;
   myMap[id2] = 4;

}

Solution 1

I guess the best way of using third-party C-structures is to use them through their friendly functions. So if you wanna use uuid_t in STL, I'd suggest you to create some kind of C++ interface/wrapper to that structure like

struct Uuid {
  uuid_t uuid;
  Uuid(const Uuid &other) { uuid_copy(uuid, other.uuid); }
  Uuid(const uuid_t other_uuid) { uuid_copy(uuid, other_uuid); }
  void generateInplace() { uuid_generate(uuid); }
  static Uuid generate() { Uuid wrapped; uuid_generate(wrapped.uuid); return wrapped; }
  bool operator<(const Uuid &other) { return uuid_compare(uuid, other.uuid) < 0; }
  bool operator==(const Uuid &other) { return uuid_compare(uuid, other.uuid) == 0; }
  // ...
};

That should hide from you the fact that uuid_t isn't structure but pointer to array (i.e. typedef unsigned char uuid_t[16]).

Note: there is boost version of uuid library

Solution 2

The STL containers always contain copies of the object, and that applies to map keys, also.

The simplest way to support this is using a custom comparator for the map.

struct UUIDComparator
{
    bool operator()(const uuid_t &a, const uuid_t &b)
    {
        //compare and return a < b
    }
};
std::map<uuid_t, int, UUIDComparator> map;

Another slightly controversial solution would be to convert the uuid_t into a std::pair<uint64_t, uint64_t> as both types are 128 bits wide, and, AFAICT, layout compatible. And the std::pair are directly usable as map keys.

std::map<std::pair<uint64_t, uint64_t>, int, UUIDComparator> map;

Solution 3

Simpler: uuid_unparse(...) converts it into a char* (37 chars long), which you can then wrap a string around...