Elements from vector change to random values - c++

Question

I am trying to solve a problem on infoarena.ro (site similar to codeforces.com, but it's in Romanian) and for some reason, some elements from a set just change to random values. Relevant code:
#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
ofstream out("test.out");
ifstream in("test.in");
struct Edge
{
int from, to, color, index;
bool erased = false, visited = false;
};
struct Event;
int point(const Event* event);
struct Event
{
int time;
bool add;
Edge *edge;
bool operator < (const Event other) const
{
return (this->time < other.time) ||
(this->time == other.time && this->add < other.add) ||
(this->time == other.time && this->add < other.add &&
point(this)>point(&other));
}
};
int point(const Event* event)
{
if(event->edge->from == event->time)
return event->edge->to;
else
return event->edge->from;
}
vector<Edge> edges;
vector<Event> events;
int main()
{
int N, M;
in >> N >> M;
for(int i = 0; i < M; i++)
{
int x, y;
in >> x >> y;
if(x > y)
swap(x, y);
Edge e = {x, y, i, i};
edges.push_back(e);
events.push_back(Event{x, true, &edges.back()});
Edge debug = *events.back().edge;
events.push_back(Event{y, false, &edges.back()});
debug = *events.back().edge;
}
sort(events.begin(), events.end());
for(Event event : events)
out << event.edge->from << " " << event.edge->to << "\n";
return 0;
}
I excluded code I wrote that is not relevant to the question.
Input:
5 6
1 2
2 5
1 4
3 1
4 3
5 3
First line is N (number of vertices) and M (number of edges). Next lines are all the edges.
Output:
44935712 44896968
1 4
1 3
44935712 44896968
3 1941924608
1 3
3 4
3 5
1 4
3 4
3 1941924608
3 5
I am trying to make a "journal" like my teacher called it. For each edge (x,y) I want to add it in a stack at stage x, and erase it at stage y (along with all other elements in the stack until I reach (x, y)). I want to sort by the "time" when I make those operations (hence "time" value in the Event struct). "Add" indicates if this is an event of adding an edge or removing it from the stack.
I am outputting the edges in the "events" vector for debugging purposes and I noticed that the values change to random stuff. Can someone explain why is this happening?

Answer 1

The problem is here
events.push_back(Event{x, true, &edges.back()});
and here
events.push_back(Event{y, false, &edges.back()});
As you push structures into the edges vector, the vector will reallocate the memory needed to store the contained structures. If such a relocation happens then all iterators and pointers to elements in the vector become invalid.
A simple solution is to store pointers in the edges vector instead, and copy the pointers for the Event structures. Another possible solution is to do two passes. One to create the edges vector, and then a separate pass (loop) to create the events vector.