title: error "expression result is unused" in c++
body: <p>I have some code like:</p>
  
  <pre><code>double calculate_self_term(double area)     {
      double corr = 2.0 * sqrtpi / sqrt(area);
      return calculate_reciprocal(0, 0, 0) + self_energy + corr;
  }
  </code></pre>
  
  <p>where <code>self_energy</code> is defined at the beginning:</p>
  
  <p><code>#define self_energy -2.0 * alpha / sqrtpi;</code></p>
  
  <p>Then I got an error:
  <strong>expression result is unused</strong> for the variable <code>corr</code>. </p>