set::find finds an element that does not exist - c++

Question

I have the following Edgeclass:
class Edge {
public:
int src, dest;
bool operator== (const Edge &edge) const {
return ((src == edge.src) && (dest == edge.dest)) || ((src == edge.dest) && (dest == edge.src));
}
bool operator<(const Edge& edge) const {
return !(((src == edge.src) && (dest == edge.dest)) || ((src == edge.dest) && (dest == edge.src)));
}
Edge(int src, int dest) {
this->src = src;
this->dest = dest;
}
};
The point of overriding the < operator is when I try to find an edge in a set Edge(0, 1) should be equal to Edge(1, 0). However, the following test code fails to do so and std::find returns an edge that doesn't even exist:
Edge edge(0, 3);
set<Edge> test;
test.insert(Edge(3, 1));
test.insert(Edge(3, 0));
auto a = test.find(edge);
cout << a->src << " " << a->dest << endl;
This will strangely print out 2 0. I can't figure out why and I'm new to C++ any help is appreciated.

Answer 1

You currently don't have a valid Compare for std::set, so your program has undefined behaviour.
Here is one that is compatible with your ==
bool operator<(const Edge& edge) const {
return std::minmax(src, dest) < std::minmax(edge.src, edge.dest);
}
This can also be used to simplify your ==
bool operator==(const Edge& edge) const {
return std::minmax(src, dest) == std::minmax(edge.src, edge.dest);
}

Answer 2

There are two issues in your code.
First, you do not check whether test.find() returns a valid edge; note that find returns end() if no element was found.
Second, your <-operator does not implement a strict ordering, it actually just defines a !=. To overcome this, I'd normalize each edge such that the lower node is always treated as the start; then decide based on the starting nodes, and only if they are equal, consider the destination nodes:
class Edge {
public:
int src, dest;
bool operator== (const Edge &edge) const {
return ((src == edge.src) && (dest == edge.dest)) || ((src == edge.dest) && (dest == edge.src));
}
bool operator<(const Edge& edge) const {
// return !(((src == edge.src) && (dest == edge.dest)) || ((src == edge.dest) && (dest == edge.src)));
int thisSrc = std::min(src,dest);
int thisDest = std::max(src,dest);
int eSrc = std::min(edge.src,edge.dest);
int eDest = std::max(edge.src,edge.dest);
if (thisSrc < eSrc) {
return true;
} else if (thisSrc > eSrc) {
return false;
} else {
return thisDest < eDest;
}
}
Edge(int src, int dest) {
this->src = src;
this->dest = dest;
}
};
#include <set>
int main() {
Edge edge(0, 3);
std::set<Edge> test;
test.insert(Edge(3, 1));
test.insert(Edge(3, 0));
auto a = test.find(edge);
if (a == test.end()) {
std::cout << "edge not found." << std::endl;
} else {
std::cout << a->src << " " << a->dest << std::endl;
}
}
Output:
3 0