for centos 7.6 , gcc-4.8 will produce some compile error for these things.
"" _csv, there must be a space between "" and _csv
constexpr to const. if not compile error occours
csv.hpp:3915:29: error:enclosing class of constexpr non-static member function ‘bool csv::CSVRow::iterator::operator==(const csv::CSVRow::iterator&) const’ is not a literal type
constexpr bool operator==(const iterator& other) const {
for centos 7.6 , gcc-4.8 will produce some compile error for these things.