title: Why it says 'push_back' has not been declared?
body: <p>Why it says 'push_back' has not been declared ?</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;vector&gt;
  using namespace std;
  int main()
  {
    vector &lt;int&gt; v(30);
    v[0].push_back(0);
   return 0;
  }
  </code></pre>