How does the == operator for multimaps behave ?

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, inta;
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, intb;
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, intc;
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.
//------------------------------------------------------------------------------

re: How does the == operator for multimaps behave ?

Nikhil.S.Ketkar@gmail.com wrote:
Yes.
Well, that is how "equality" is defined for containers. If you think there
is a difference, maybe you want to explain what you mean by "properly
equal" (as opposed to just "equal").

Anyway, table 65 (Container requirements) defines the semantics of "==" for
all containers as

a == b
if and only if
a.size() == b.size() and equal( a.begin(), a.end(), b.begin(), b.end() )


This output is correct.

BTW: what is the motivation to use such an indirect way of specifying the
elements of the maps (by using the array "numbers")?



Best

Kai-Uwe Bux

re: How does the == operator for multimaps behave ?

On 2008-08-30 06:46, Nikhil.S.Ketkar@gmail.com wrote:Yes, it checks if the two multimaps are equal, and two multimaps are
equal if all the contained elements are equal. Perhaps you wanted to
check for identity, if two references/pointers to multimaps referes to
the same multimap? In that case compare the addresses of the objects
referred to.

--
Erik Wikström