title: How to append an attribute-value pair on an existing json11 object (c++)?
body: <p>For example,
  I'm building a json message using following code:</p>
  
  <pre><code>json11::Json my_json = json11::Json::object{
      { "key_val1", val1},
      { "key_val2", val2},
      { "key_val3", val3},
      { "key_val4", val4 }
  };
  
  std::string message = my_json.dump();
  </code></pre>
  
  <p>But if i want to have this json11 object contain different attribute-value pair based on some condition then I've to repeat the same code multiple times.</p>
  
  <p>Is there any way to append attribute-value pair to an existing json11 object?
  So that i can build a base object and then append necessary attributes on demand.</p>