How does the == operator for multimaps behave ?

From:

Nikhil.S.Ketkar@gmail.com

Newsgroups:

comp.lang.c++

Date:

Fri, 29 Aug 2008 21:46:55 -0700 (PDT)

Message-ID:

<061b13c0-46a7-4dfb-b2bf-4d306abf2f17@k36g2000pri.googlegroups.com>

Hi,

How does the == operator for multimap in STL behave ? I was under the
impression that this is supposed to properly compare multimaps for
equality. It seems that what it actually does is just check if each
member in location L in one multimap is equal to the the member in
location L in the other multimap. The documentation on
http://www.sgi.com/tech/stl/Multimap.html says "Tests two multimaps
for equality. This is a global function, not a member function." and
nothing more.

Am I missing something ?
I have added an illustrative program with output.

Thanks,
Nikhil

#include <iostream>
#include <map>

int main(int argc, char *argv)
{
  int numbers[] = {1,2,3,4,5};

  std::multimap<int, int> a;
  a.insert(std::pair<int, int>(numbers[0], numbers[2]));
  a.insert(std::pair<int, int>(numbers[0], numbers[4]));
  a.insert(std::pair<int, int>(numbers[1], numbers[3]));
  a.insert(std::pair<int, int>(numbers[2], numbers[2]));
  a.insert(std::pair<int, int>(numbers[2], numbers[4]));

  std::multimap<int, int> b;
  b.insert(std::pair<int, int>(numbers[0], numbers[2]));
  b.insert(std::pair<int, int>(numbers[0], numbers[4]));
  b.insert(std::pair<int, int>(numbers[1], numbers[3]));
  b.insert(std::pair<int, int>(numbers[2], numbers[2]));
  b.insert(std::pair<int, int>(numbers[2], numbers[4]));

  std::cout << "Multimap a and b were found to be ";
  if (a == b)
    std::cout << "equal.\n";
  else
    std::cout << "unequal.\n";

  std::multimap<int, int> c;
  c.insert(std::pair<int, int>(numbers[0], numbers[4]));
  c.insert(std::pair<int, int>(numbers[0], numbers[2]));
  c.insert(std::pair<int, int>(numbers[1], numbers[3]));
  c.insert(std::pair<int, int>(numbers[2], numbers[2]));
  c.insert(std::pair<int, int>(numbers[2], numbers[4]));

  std::cout << "Multimap a and c were found to be ";
  if (a == c)
    std::cout << "equal.\n";
  else
    std::cout << "unequal.\n";

  return 0;
}

// Output
//------------------------------------------------------------------------------
// Multimap a and b were found to be equal.
// Multimap a and c were found to be unequal.
//------------------------------------------------------------------------------