problem about reference to vector element

[breadbread1984]

I met a problem when doing my project. I abstract the problem to the following code.

#include <iostream>
#include <vector>
#include <boost/numeric/ublas/matrix.hpp>
#include <boost/numeric/ublas/io.hpp>
#include <boost/numeric/ublas/matrix_proxy.hpp>

using namespace std;
namespace ublas = boost::numeric::ublas;

int main()
{
	vector<ublas::matrix<double> > m;
	m.push_back(ublas::matrix<double>());
	ublas::matrix<double> & n = m.back();
	m.push_back(ublas::matrix<double>());
	n = ublas::scalar_matrix<double>(3,3);
	for(vector<ublas::matrix<double> >::iterator it = m.begin() ; it != m.end() ; it++)
		cout<<*it<<endl;
}

The code can be compiled correctly. But running the executable will cause a segment fault. If line 15 of the code is removed, the executable works fine. What may be the problem. Thanks

[JLBorges]

ublas::matrix<double> & n = m.back();
m.push_back(ublas::matrix<double>());
// the reference 'n' is no longer a valid reference
// (the sequence might have been relocated in memory)