Question

Is there a Java Map keySet() equivalent for C++’s std::map?

The Java keySet() method returns “a set view of the keys contained in this map.”

Answer 1

Perhaps the following might be of use:

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

template< class Key, 
          class T, 
          class Comparator,
          class MapAllocator,
          class SetAllocator>
void make_key_set(const std::map<Key,T,Comparator,MapAllocator>& map, 
                  std::set<Key,Comparator,SetAllocator>& set)
{
   set.clear();
   typedef typename std::map<Key,T,Comparator,MapAllocator> map_type;
   typename map_type::const_iterator itr = map.begin();
   while (map.end() != itr)
   {
      set.insert((itr++)->first);
   }
}

int main()
{
  std::map<std::string, double> m;

  m["one"] = 1.1;
  m["two"] = 2.2;
  m["three"] = 3.3;

  std::set<std::string> key_set;

  make_key_set(m,key_set); 

  std::copy(key_set.begin(), key_set.end(),
            std::ostream_iterator<std::string>(std::cout, "\n"));

  return  0;
}

An overload for the make_key_set function taking STL compatible sequences such as std::vector, std::deque or std::list can be as follows:

template< class Key, 
          class T, 
          class Comparator,
          class MapAllocator,
          class SeqAllocator,
          template<class,class> class Sequence>
void make_key_set(const std::map<Key,T,Comparator,MapAllocator>& map, 
                  Sequence<Key,SeqAllocator>& sequence)
{
   sequence.clear();
   typedef typename std::map<Key,T,Comparator,MapAllocator> map_type;
   typename map_type::const_iterator itr = map.begin();
   while (map.end() != itr)
   {
      sequence.push_back((itr++)->first);
   }
}