title: How do I show the value of a #define at compile-time?
body: <p>I am trying to figure out what version of Boost my code thinks it's using. I want to do something like this:</p>
  
  <p><code>#error BOOST_VERSION</code></p>
  
  <p>but the preprocessor does not expand BOOST_VERSION.</p>
  
  <p>I know I could print it out at run-time from the program, and I know I could look at the output of the preprocessor to find the answer. I feel like having a way of doing this during compilation could be useful.</p>