title: Equation Parsing Library C++
body: <p>I'm sure there must be something like this somewhere but I can't seem to find anything useful on here or Google. I had hoped Boost might have something but alas not. </p>
  
  <p>What I'm after is a lightweight library that can take a string from the user, for example <code>"y=2x+3"</code> and parse it returning an object or function which returns <code>y</code> when given <code>x</code>.</p>
  
  <p>Can anybody recommend something for this? <em>(Worst case I could write one myself but no point reinventing the wheel and all.)</em></p>
  
  <p>Things which can be assumed if necessary;</p>
  
  <ul>
  <li>Preset variable names</li>
  <li>Number of variables fixed</li>
  </ul>