title: Why using namespace std is necessary here?
body: <pre><code>#include &lt;iostream&gt;
  using namespace std;
  
  int main() {
      cout &lt;&lt; "!!!Hello World!!!" &lt;&lt; endl; // prints !!!Hello World!!!
      return 0;
  }
  </code></pre>
  
  <p>If I remove the 2nd statement,the build will fail.</p>
  
  <p>Why is it necessary?</p>