doc/snippets/Map_placement_new.cpp

int data[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
Map<RowVectorXi> v(data, 4);
cout << "The mapped vector v is: " << v << "\n";
new (&v) Map<RowVectorXi>(data + 4, 5);
cout << "Now v is: " << v << "\n";