Create std::set with keys of a std::map

[treuss]

Hi,

title says it all. Is there an easy way to create a set that contains the keys of my map? Background is, that one module of my program (a parser) needs the keys but I don't want to make the class that forms the second object of my map known to this module.

[jwbarton]

You could use something like:

#include <string>
#include <map>
#include <set>
#include <algorithm>


template <class MapType>
const typename MapType::key_type& GetMapKey( typename MapType::value_type& entry )
{
   return entry.first;
}

int main()
{
   typedef std::map<std::string,int> TestMap;
   TestMap testMap;

   testMap.insert( TestMap::value_type("A",1) );
   testMap.insert( TestMap::value_type("B",2) );
   testMap.insert( TestMap::value_type("C",3) );
   testMap.insert( TestMap::value_type("D",4) );

   std::set<TestMap::key_type> testSet;

   std::transform( testMap.begin(), testMap.end(), std::inserter(testSet,testSet.end()), GetMapKey<TestMap> );

   return 0;
}

The std::transform() inserts each key of the map into the set, using the templated function GetMapKey() to extract the key from the map entries.

[kuphryn]

one solution is pointer. you will better manage memory.

Kuphryn