title: C++ - Optional arguments by reference
body: <p>I have an exam on C++ coming up, and I'm solving a few ones from past years. I have this question in one of them:</p>
  
  <blockquote>
    <p>A function calculates the volume of a
    prysm. Arguments passed are height,
    depth and width. Arguments and return value are doubles
    Depth is optional and
    should default to 10. 
    Hypothesis 1:
    All parameters are passed by value</p>
  </blockquote>
  
  <p>I answered <code>double volume_prysm(const double width, const double height, const double depth = 10);</code></p>
  
  <blockquote>
    <p>Hypothesis 2:
    All parameters are passed by reference</p>
  </blockquote>
  
  <p>How can I define a reference parameter in order for it to default to 10?</p>
  
  <p>Thanks for your time!</p>
  
  <p>PS: Sorry y'all for not translating</p>