How to contain my class with std::set

Question

I'm trying to write a program in C++ language.

Class Edge indicates the connection between u and v.

Edge a which indicates the connection between u and v. Edge a' which indicates the connection between v and u. Edge a and a' indicate same connection. So, I want to contain either a or a'.

I know "set" stores unique elements. So I want to use this. I define operator< below.

When I search bugs, I found some trubles. I store (1,2) -> (1,2) -> (2,1) -> (3,2) -> (2,3) -> (5,2).

But set stores

1 2
5 2
3 2
1 2 <-- Why ????

Could you help me??

#include<iostream>
#include<set>

class Edge {

private:
  int u, v;

public:
  bool operator< (const Edge& e) const {
    bool result = true;
    if( (u == e.u && v == e.v) || (v == e.u && u == e.v) ) {
      result = false;
    }
    return result;
  }

  std::pair<int, int> pair() const {
    return std::pair<int, int>(u, v);
  }

  Edge(int u_, int v_) : u(u_), v(v_) {}
};

int main(void) {
  std::set<Edge> edge;
  std::set<Edge>::iterator eit;

  edge.insert(Edge(1,2)); // <-- (1,2) can be contained.
  edge.insert(Edge(1,2)); // <-- (1,2) doesn't have to be contained.
  edge.insert(Edge(2,1)); // <-- (2,2) doesn't have to be contained.

  edge.insert(Edge(3,2)); // <-- (3,2) can be contained.
  edge.insert(Edge(2,3)); // <-- (2,3) doesn't have to be contained.
  edge.insert(Edge(5,2)); // <-- (5,2) doesn't have to be contained.

  edge.insert(Edge(1,2)); // <-- (1,2) doesn't have to be contained. But edge contains this. Why?

  for(eit = edge.begin(); eit != edge.end(); eit++) {

    std::cout << (*eit).pair().first << " " << (*eit).pair().second << std::endl;
  }

  return 0;
}

Answer 1

Your operator< is testing for equality, not less-than. Try:

if (u < e.u)
    result = true;
else if (e.u < u)
    result = false;
else
    result = (v < e.v);

Edit: According to the comment I misunderstood the question - the set is supposed to reject a duplicate in either order. The comparison operator needs to be consistent, so here's one that might work.

if (min(u,v) < min(e.u,e.v))
    result = true;
else if (min(e.u,e.v) < min(u,v))
    result = false;
else
    result = (max(u,v) < max(e.u,e.v));

Answer 2

You operator < implementation is more like an equality implementation. try doing lexical less than implementation:

bool operator< (const Edge& e) const
{
    return (u < e.u) || (u == e.u && v < e.v);
}