C++: STL multimap.equal_range()

Question

I've got this code and I cannot understand part where equal_range method returns iterators. I know range is pair object with two multimap objects inside, but what I don't get, is why there is 'for (it = range.first; it != range.second; ++it)' - What does this mean exactly ?

// multmap.cpp -- use a multimap
#include <iostream>
#include <string>
#include <map>
#include <algorithm>

typedef int KeyType;
typedef std::pair<const KeyType, std::string> Pair;
typedef std::multimap<KeyType, std::string> MapCode;

int main()
{
using namespace std;
MapCode codes;
codes.insert(Pair(415, "San Francisco"));
codes.insert(Pair(510, "Oakland"));
codes.insert(Pair(718, "Brooklyn"));
 codes.insert(Pair(718, "Staten Island"));
  codes.insert(Pair(415, "San Rafael"));
  codes.insert(Pair(510, "Berkeley"));

  cout << "Number of cities with area code 415: "
    << codes.count(415) << endl;
  cout << "Number of cities with area code 718: "
    << codes.count(718) << endl;
  cout << "Number of cities with area code 510: "
    << codes.count(510) << endl;
  cout << "Area Code City\n";

  MapCode::iterator it;
  for (it = codes.begin(); it != codes.end(); ++it)
  cout << " " << (*it).first << " "
  << (*it).second << endl;

  pair<MapCode::iterator, MapCode::iterator> range
        = codes.equal_range(718);

  cout << "Cities with area code 718:\n";
  for (it = range.first; it != range.second; ++it) //<------------------ here
  cout << (*it).second << endl;
    return 0;
}

Answer 1

The result of equal_range , namely your range object, is a pair of two iterators [beginning-of-range, end-of-range) . So you want to iterate over [range.first, range.second) :

auto range = m.equal_range(4);

+---+---+---+---+---+---+---+---+---+
| 2 | 3 | 3 | 4 | 4 | 4 | 4 | 5 | 6 |    =:   m
+---+---+---+---+---+---+---+---+---+
            ^               ^
            |               |
       range.first    range.second

Answer 2

The iterators in the pair define the range of items with keys equal to what you searched for in the manner [range.first, range.second) .

This means that to iterate over that range, you start from range.first and advance the iterator until it reaches range.second , which means that you have just stepped off the equal range. Conceptually it's the same as what happens when you iterate over a range [container.begin(), container.end()) .

Answer 3

equal_range returns a pair of iterators i1, i2 such that all elements within the range [i1, i2) have the same key. Hence, in order to iterate through all cities with code 718, you call equal_range , and then iterate from the returned pair's first to the returned pair's second .