Modern JSON C++: Reading and Writing complex data

I am using Modern C++ JSON to encode and decode JSON.

The examples and documentation while extensive do have a few holes particularly around extracting the data backout from the JSON data.

The following example shows how to encode a complex JSON object:

struct Point
{
  double x_, y_, z_;
};
 
nlohmann::json object = nlohmann::json::object();
{
  /// Create JSON object with data
  std::vector<Point> pnts_ = { { 0, 1, 2}, { 3, 4, 5} };
  int id = 1;
  std::string desc( "description" );
  int tid = -1;
 
  object["fp"]["id"] = id;
  object["fp"]["desc"] = desc;
  object["fp"]["tid"] = tid;
 
  nlohmann::json points = nlohmann::json::array();
  for ( auto pnt : pnts_ )
  {
    nlohmann::json point = nlohmann::json::object();
    point["x"] = pnt.x_;
    point["y"] = pnt.y_;
    point["z"] = pnt.z_;
    points.push_back( point );
  }
  object["points"] = points;
  object["size"] = pnts_.size();
}

The following shows how to decode the object created:


{
  /// Read JSON object
  const auto& itr = object.find( "fp" );
  if ( itr == object.end() )
  {
    /// error
    return false;
  }
  /// TODO do not assume the data is well formed.
 
  auto obj = itr.value();
  auto idItr = obj.find( "id" );
  int id = idItr->get<int>();
 
  auto descItr = obj.find( "desc" );
  std::string desc = descItr->get<std::string>();
 
  auto trackItr = obj.find( "tid" );
  int tid = trackItr->get<int>();
 
  auto sizeItr = object.find( "size" );
  int size( sizeItr->get<int>() );
 
  auto pointsItr = object.find( "points" );
  const auto& points = pointsItr.value();

  std::vector<Point> pnts_;
  pnts_.reserve( size );
  for ( auto pntItr : points.items() )
  {
    const auto& pnt = pntItr.value();
 
    Point point;
    point.x_ = pnt.find( "x" )->get<double>();
    point.y_ = pnt.find( "y" )->get<double>();
    point.z_ = pnt.find( "z" )->get<double>();
    pnts_.push_back( point );
  }
}




The following shows JSON object we created:


{

"fp":

{"desc":"description",

"id":1,

"tid":-1},

"points":[{"x":0.0,"y":1.0,"z":2.0},{"x":3.0,"y":4.0,"z":5.0}],

"size":2

}