Good evening!

Why i'm getting the following error message? How can i fix it?
Qt Code: 
  1. QList<Number> Historic::descendingOrder(QList<Number> ranking)
  2. {
  3.     Number aux(0, 0);
  4.     int length = ranking.size( );
  5.  
  6.     for(int i = 0; i < length; i++)
  7.     {
  8.         for(int j = 0; j < (length - 1); j++)
  9.         {
  10.             if(ranking.at( j ).acum < ranking.at(j + 1).acum)
  11.             {
  12.                 aux.val = ranking.at(j + 1).val;
  13.                 aux.acum = ranking.at(j + 1).acum;
  14.                 ranking.at(j + 1).val = ranking.at( j ).val; //Error message here
  15.                 ranking.at(j + 1).acum = ranking.at( j ).acum; //Error message here
  16.                 ranking.at( j ).val = aux.val; //Error message here
  17.                 ranking.at( j ).acum = aux.acum; //Error message here
  18.             }
  19.         }
  20.     }
  21.  
  22.     return ranking;
  23. }
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D:\Programming\C-C++\Qt\Windows\historic.cpp:121: error: assignment of member 'Number::val' in read-only object
ranking.at(j + 1).val = ranking.at( j ).val;
Everything in "Number" class is public. I don't understand!