template <class T>

boost::unordered_map<std::string, vector_expression <T> > c;

vector<int> v (10);

for (unsigned i = 0; i < v.size (); ++ i)

v (i) = i;

c <int>["first"] = v;

error: no match for ‘operator=’ (operand types are ‘boost::unordered::unordered_map<std::__cxx11::basic_string<char>, boost::numeric::ublas::vector_expression<int>, boost::hash<std::__cxx11::basic_string<char> >, std::equal_to<std::__cxx11::basic_string<char> >, std::allocator<std::pair<const std::__cxx11::basic_string<char>, boost::numeric::ublas::vector_expression<int> > > >::mapped_type {aka boost::numeric::ublas::vector_expression<int>}’ and ‘boost::numeric::ublas::vector<int>’)

   columns <int>["first"] = v;