title: How to input 2D vector correctly?
body: <p>When I try to set and populate a 2D vector as follows, I couldn't input <code>int</code> numbers to them.
  I try to input numbers usinga range-based for loop; but it doesn't finish after the expected number of loop cycles.</p>
  <pre><code>#include &lt;iostream&gt;
  #include &lt;vector&gt;
  using namespace std;
  
  int main(){
      int N;
      vector&lt;vector&lt;int&gt;&gt; A(N,vector&lt;int&gt;(6)); 
      cin&gt;&gt;N;
      for(auto row_vec:A) {
          for(auto x:row_vec) cin&gt;&gt;x;
      }
      return 0;
  }
  </code></pre>
  <p>What is wrong with that code?</p>
  <p>The following is my test input on the console:</p>
  <pre><code>[ec2-user@ip-10-0-1-187 atcoder]$ ./052
  2
  1 2
  2 4
  2 4
  </code></pre>