g++ compile error with std::u16string?

I have a hard time getting std::u16string to compile in g++-4.8 or g++-5.1.
I'm using ubuntu 14.04. I keep getting errors like undefined u16string reference
or u16string is not a member of std.

#include<iostream>
#include<string>
using namespace std;    
int main(){ 
     u16string str = u"hello u16string";
     return 0;
}

And then compile with g++ -std=c++14 main.cpp or g++ -std=c++11 main.cpp

update

the above code works fine in g++4.8. i was missing something else.