title: How do I check whether character constants conform to ASCII?
body: <p>A comment on an earlier version of <a href="https://stackoverflow.com/questions/42055919/how-do-i-take-in-a1-style-rowcolumn-specifications-as-input/42055958#42055958">this answer</a> of mine alerted me to the fact that I can't assume that <code>'A'</code>, <code>'B'</code>, <code>'C'</code> etc. have successive numeric values. I had sort of assumed the C or C++ language standards guarantee that this is the case.</p>
  
  <p>So, how should I determine whether consecutive letter characters' values are themselves consecutive? Or rather, how can I determine whether the character constants I can express within single quotes have their ASCII codes for a numeric value?</p>
  
  <p>I'm asking how to do this both in C and in C++. Obviously the C way would work in C++ also, but if there's a C++ish facility for doing this I'm interested in that as well. Also, I'm asking about the newest relevant standards (C11, C++17).</p>