is there an iterator across unique keys in a std::multimap?

Question

Is there a simple or standard way to have a multimap iterator which iterate across unique keys in a multimap?

i.e. for a set that looks like: {1, \"a\"}, {1, \

Answer 1

if you have to pass over all unique keys quickly then you can use std::map instead;

typedef std::map< KeyType, std::list< ValueType > > MapKeyToMultiValue;

Insertion would be more difficult, However you can iterate over all keys without having to bother with duplicate entries. Insertion would look as follows:

void insert_m(MapKeyToMultiValue &map, const KeyType key, const ValueType value )
{
  auto it = map.find( key );
  if (it == map.end())
  {
     std::list empty;
     std::pair< MapKeyToMultiValue::iterator, bool > ret =
        map.insert( MapKeyToMultiValue::value_type( key, empty ) );
     it = ret.first;
  }

  it->second.push_back( value );
}

or you can make that very templated:

template > >
void insert_multi( MapType &map, const KeyType key, const ValueType value )
{

  auto it = map.find( key );
  if (it == map.end())
  {
     std::list empty;
     std::pair< typename MapType::iterator, bool > ret =
        map.insert( typename MapType::value_type( key, empty ) );
     it = ret.first;
  }

  it->second.push_back( value );
}

The full test program looks as follows:

#include