title: Rcpp::DataFrame - find column type
body: <p>In all Rcpp examples that I could find, one must know the type of DataFrame columns, then you can extract column into vector, e.g.:</p>
  
  <pre><code>// construct the data.frame object
  Rcpp::DataFrame DF = Rcpp::DataFrame(Dsexp);
  // and access each column by name
  Rcpp::IntegerVector a = DF["a"];
  Rcpp::CharacterVector b = DF["b"];
  Rcpp::DateVector c = DF["c"];
  </code></pre>
  
  <p>In R, one could find column types using <code>sapply()</code> function and <code>class</code> or <code>typeof</code>.</p>
  
  <p>However, I couldn't find the external API in Rcpp responsible for type checking. Is there a way to find column type programmatically?</p>