Here's what I have tried:

Header:
Qt Code: 
  1. #include <QQuickItem>
  2. #include <iostream>
  3.  
  4. class Controller : public QObject
  5. {
  6.     Q_OBJECT
  7.  
  8.     Q_PROPERTY(QList <QVariantList> names READ names WRITE setnames NOTIFY namesChanged)
  9.     QList <QVariantList> m_names;
  10.  
  11. public:
  12.     Controller()
  13.     {
  14.     }
  15.     ~Controller() {
  16.     }
  17.  
  18.     QList <QVariantList> names() const
  19.     {
  20.         return m_names;
  21.     }
  22.  
  23. public slots:
  24.     void setnames(QList <QVariantList> arg)
  25.     {
  26.         QVariantList p;
  27.         if (arg.size () > 0)
  28.         {
  29.             p = arg.first ();
  30.             std::cout << "\narg: \n" << p[0].toInt ();
  31.         }
  32.         else
  33.             std::cout << "\nqqqq " << arg.size () << "\n";
  34.     }
  35.  
  36. signals:
  37.     void namesChanged(QList <QVariantList> arg);
  38. };
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qml

Qt Code: 
  1. import QtQuick 2.0
  2. import FromCpp 1.0
  3.  
  4. Rectangle
  5. {
  6.     property variant arras: [[1,2,3], [4,5,6]]
  7.     Controller
  8.     {
  9.         id: ppp
  10.     }
  11.  
  12.     MouseArea
  13.     {
  14.         anchors.fill: parent
  15.         onClicked:
  16.         {
  17.             ppp.setnames(arras)
  18.             console.log(arras.length)
  19.         }
  20.     }
  21. }
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The exact output from QtCreator:
Qt Code: 
  1. Starting /home/***/documents/test/build-junk-Desktop_Qt_5_1_0_GCC_64bit-Debug/junk...
  2. QML debugging is enabled. Only use this in a safe environment.
  3. 2
  4.  
  5. qqqq 0
  6. QThreadStorage: Thread 0x181e270 exited after QThreadStorage 2 destroyed
  7. /home/***/documents/test/build-junk-Desktop_Qt_5_1_0_GCC_64bit-Debug/junk exited with code 0
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///

Here, as you can see the size of the 2D array from QML is printed correctly as 2 and the size is printed 0 from c++.

Why is that happening? Please explain.