title: Rows of Rcpp::DataFrame
body: <p>I have this code</p>
  
  <pre><code> // [[Rcpp::export]]
  void testDf(Rcpp::DataFrame pq)
  {
    Rcpp::NumericVector p = pq[1];
    std::cout&lt;&lt;p[2]&lt;&lt;std::endl;
  }
  </code></pre>
  
  <p>Which is the right way of accessing an element of <strong>virtual</strong> row of a particular column . But if I do this </p>
  
  <pre><code>// [[Rcpp::export]]
  void testDf(Rcpp::DataFrame pq)
  {
    std::cout&lt;&lt;pq[1][2]&lt;&lt;std::endl;
   }    
  </code></pre>
  
  <p>It return to me an error that <strong>operator[] is ambiguous</strong> . 
  But if we see, DataFrame is just a set of vectors. From the Data structure point of view DataFrame will hold <strong>pointers</strong> to set of vectors. So the <strong>[]</strong> operator will return me a pointer and I can further access the pointer's different element through dereferencing it. 
  Please correct me if I am wrong. Just started with the awesome Rcpp package. I am having few theoretical questions like this. I will be grateful for the answers.   </p>