can't resolve std::unordered_set in Xcode

When I tried

int main(){
    std::unordered_set <int> s;
}

According to (unordered_set)[http://www.cplusplus.com/reference/unordered_set/unordered_set/?kw=unordered_set], it is under std::

No template named 'unordered_set'; did you mean 'unordered_map'?