c++ - invalid operator < while sorting std::list

Question

I have a std::list graph edges and i want to sort the edges based on their destination outdegree and then their indegree. But i am getting getting exception of invalid operator < during my comparison function below is my code. My list contains the pointers to the edges and edges have destination nodes as their member.

bool compareEdges(const Edge  *e1,const Edge *e2){
if(e1->destination->outdegree < e2->destination->outdegree){
    return true;
}
else if(e1->destination->outdegree > e2->destination->outdegree){
    return false;
}
else if(e1->destination->indegree > e2->destination->indegree){
        return false;
    }
return true;

}

And here is the call to the sort function.

currentNode->edgeList.sort(compareEdges);

Please help me in removing this exception.

Thanks

Answer 1

Your comparator returns true when both relevant fields are equal. This is invalid, so it may well be what the sort implementation has detected via assert.

You're supposed to pass a "less than" predicate to sort: formally a "strict weak order". Anything else is undefined behavior. It seems in this case you got lucky, and the implementation detects that it has got into an impossible situation due to inconsistent comparisons.