OOP - How to demand derived classes to set a value to base attribute?
<p>The output of my program is a table in a file.</p>

<p>Some users like the output as Excel file, the others prefer CSV, and so on.<br>
  I therefore have an abstarct class <code>Document</code> with 2 derived classes:<br>
  <code>ExcelDocument</code> and <code>CsvDocument</code>.  </p>

<p>The delimiter between the output results is different in each class:<br>
In the CSV file the delimiter is <code>,</code> and in the Excel the delimiter is <code>/t</code>.  </p>

<p>How can I force the derived classes to set value to the <code>delimiter</code> attribute of the abstract <code>Document</code> class?</p>