Pair equal operator overloading for inserting into set - c++

Question

I am trying to add a pair<int,int> to a set. If a pair shares the same two values as another in the set, it should not be inserted.
Here's my non-working code:
typedef std::pair<int, int> PairInt;
template<>
bool std::operator==(const PairInt& l, const PairInt& r)
{
return (l.first == r.first && l.second == r.second) ||
(l.first == r.second && l.second == r.first);
}
int main()
{
std::set<PairInt> intSet;
intSet.insert(PairInt(1,3));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(4,1));
}
At the moment, the (4,1) pair gets added even though there is already a (1,4) pair. The final contents of the set are:
(1 3)
(1 4)
(4 1)
and I want it to be
(1 3)
(1 4)
I've tried putting breakpoints in the overloaded method, but they never get reached. What have I done wrong?

Answer 1

Sets are based on operator< (an ordering/equivalence relationship), not operator== (which is an equality relationship).
To do the thing that you are trying to do, use a custom comparator:
#include <set>
#include <utility>
#include <cassert>
typedef std::pair<int, int> PairInt;
PairInt normalize(const PairInt& p) {
return p.second < p.first ? PairInt(p.second, p.first) : p;
}
struct Comparator {
bool operator()(const PairInt& l, const PairInt& r) const {
//Compare canonical forms of l and r.
return normalize(l) < normalize(r);
}
};
int main()
{
std::set<PairInt, Comparator> intSet;
intSet.insert(PairInt(1,3));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(4,1));
assert(intSet.size() == 2);
}

Answer 2

You will need to provide a comparison function for seeing of one item is less than the other, not for determining if they are equal. Here is a complete example:
#include <utility>
#include <algorithm>
#include <set>
#include <iostream>
typedef std::pair<int, int> PairInt;
typedef bool Compare(const PairInt &,const PairInt &);
bool compare(const PairInt &l,const PairInt &r)
{
int lfirst = std::min(l.first,l.second);
int rfirst = std::min(r.first,r.second);
if (lfirst<rfirst) return true;
if (rfirst<lfirst) return false;
return std::max(l.first,l.second)<std::max(r.first,r.second);
}
int main()
{
typedef std::set<PairInt,Compare*> IntSet;
IntSet intSet(compare);
intSet.insert(PairInt(1,3));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(1,4));
intSet.insert(PairInt(4,1));
for (IntSet::const_iterator i=intSet.begin(); i!=intSet.end(); ++i) {
std::cerr << i->first << "," << i->second << "\n";
}
}
Output:
1,3
1,4

Answer 3

The compare should determine if first item is less than the second item. So it should be like this:
namspace std
{
template<>
bool operator < (const PairInt& l, const PairInt& r)
{
//swap only if they're unequal to avoid infinite recursion
if (l.first != l.second)
{
//swap elements, considering your special case
if (l.first == r.second && l.second == r.first)
return l < PairInt(r.second, r.first); //call again!
}
//actual comparison is done here
if ( l.first != r.first )
return l.first < r.first;
else
return l.second < r.second;
}
}
Now it gives the desired output:
1,3
1,4
Have a look at the online demo.
Note that the compare function follows : Strict weak ordering