title: using std::<type> v.s. using std namespace
body: <p>Two ways to use the using declaration are</p>
  
  <pre><code>using std::string;
  using std::vector;
  </code></pre>
  
  <p>or</p>
  
  <pre><code>using namespace std;
  </code></pre>
  
  <p>which way is better?</p>