title: C++: Reading a json object from file with nlohmann json
body: <p>I am using the nlohmann's json library to work with json objects in c++. Ultimately, I'd like to read a json object from a file, e.g. a simple object like this.</p>
  
  <pre><code>{
  "happy": true,
  "pi": 3.141
  }
  </code></pre>
  
  <p>I'm not quite sure how to approach this. At <a href="https://github.com/nlohmann" rel="noreferrer">https://github.com/nlohmann</a> several ways are given to deserialise from a string literal, however it doesn't seem trivial to extend this to read in a file. Does anyone have experience with this? </p>