#include <iostream>
#include <type_traits>
#include <vector>

template< class T, typename = std::enable_if_t< std::is_same< T, std::vector< double >>::value >>
std::ostream& operator<< (std::ostream& os, const T& v)
{
    for (auto it = std::begin (v); it < std::end (v); ++it) {
        os << *it;
        if (it + 1 < std::end (v)) {
            os << ',';
        }
    }
   return os;
}

int main()
{
    std::vector< double > v (10, 1.);
    std::cout << v << "n";
}

#include <iostream>
#include <type_traits>
#include <vector>

template< template< class > class T, typename = std::enable_if_t< std::is_same< T< double >, std::vector< double >>::value >>
std::ostream& operator<< (std::ostream& os, const T< double >& v)
{
    for (auto it = std::begin (v); it < std::end (v); ++it) {
        os << *it;
        if (it + 1 < std::end (v)) {
            os << ',';
        }
    }
   return os;
}

int main()
{
    std::vector< double > v (10, 1.);
    std::cout << v << "n";
}