[C++] Vector (push_back); g++ -O2; Segmentation fault

Question

I'm having problem with vector, (in the usage of push_back) but it only appears when using additional g++ flag -O2 (I need it).

#include <cstdio>
#include <vector>

typedef std::vector<int> node;
typedef std::vector<node> graph;

int main()
{
    int n, k, a, b, sum;
    bool c;
    graph g(n, node());
    c = scanf("%i%i", &n, &k);

    for(int i=0; i<n; i++)
    {
        sum=2;
        for(int j=0; j<i; j++)
            sum*=2;
        for(int j=0; j<sum; j++)
        {
            if(j%2==0)
                c = scanf("%i", &a);
            else
            {

                c = scanf("%i", &b);
                a += b;
                g[i].push_back(a); //---------------LINE WHICH CAUSES SEGMENTATION FAULT
            }

        }
    }

    for(int i=n-2; i>=0; i--)
    {
        for(size_t j=0; j<g[i].size(); j++)
        {
            if(g[i+1][(j*2)] >= g[i+1][(j*2)+1])
                g[i][j] = g[i+1][j*2];
            else
                g[i][j] = g[i+1][(j*2)+1];
        }
    }

    printf("%i\n", g[0][0]);

    return 0;
}

Answer 1

Initializing the vector with n before the input operation means you're invoking the dreaded Undefined Behavior. As stated here, the program is allowed to do anything after that.

Answer 2

I think you have:

graph g(n, node());
c = scanf("%i%i", &n, &k);

in the reverse order. As it stands, the variable 'n' which you use to size graph is not initialised.

Answer 3

Works perfectly if you initialize n as I already mentioned in my comment. Change the first lines to:

int n, k, a, b, sum;
int c;
c = scanf("%i%i", &n, &k); // initialize n *first*
if(c != 2) return -1; // scanf does not return bool but the number of parsed arguments
graph g(n, node());