title: `JSONcpp` C++ library to modify JSON files
body: <p>I was looking for JSON C++ libraries and the <code>JSONcpp</code> library seems the most used.</p>
  
  <p>I would like to use for the following problem.
  Basically suppose I have a JSON file like:</p>
  
  <pre><code>{"data" : [
    {"id": 1, "val" : [
        {"id":"el1", "x": val1, "y" : val2}, 
        {"id":"el2", "x": val3, "y" : val4}
        ],
    {"id": 2, "val" : [
          {"id":"el1", "x": val5, "y" : val6},
          {"id":"el2", "x": val5, "y" : val8},
          {"id":"el3", "x": val7, "y" : val10}
    ]
    {"id": 3, "val" : [
          {"id":"el1", "x": val1, "y" : val2},
          {"id":"el2", "x": val1, "y" : val2},
          {"id":"el3", "x": val1, "y" : val2},
          {"id":"el4", "x": val1, "y" : val2},
          {"id":"el5", "x": val1, "y" : val2}
    ]
    ....
  }
  </code></pre>
  
  <p>The first run of the interactive C++ program generate the file. Next times the program is run at first it loads the JSON values. The list represented by the outer array can be modified by adding an element, removing an element or modifying one and the same for the inner elements.</p>
  
  <p>At each run I want to modify the file.
  Appending an element to the file should be simple. But does the library allow to modify only one single element of the array (so only some lines of the file)? Or each time the file should be/is deleted and created anew?</p>