Meta Types and QVariant

[kroenecker]

I'm trying to implement my own model/delegate where the model returns pointers to a QHash<QString, QString> *. I've called Q_DECLARE_METATYPE in order to register the new type and have also called qRegisterMetaType<MyType>("MyType"). Unfortunately, when I try to convert the call to index.data(Qt:isplayType) into <QHash<QString, QString> *, all I seem to get back is a null pointer...

typedef QHash<QString, QString>* MyType; 
Q_DECLARE_METATYPE(MyType)
 
class MyDelegate : public QItemDelegate
{
  Q_OBJECT
 
 public:
  MyDelegate(QWidget *parent = 0);
 
  virtual void paint(QPainter *painter, const QStyleOptionViewItem &option, 
		     const QModelIndex &index) const;
};

typedef QHash<QString, QString>* MyType; 
Q_DECLARE_METATYPE(MyType)

class MyDelegate : public QItemDelegate
{
  Q_OBJECT

 public:
  MyDelegate(QWidget *parent = 0);

  virtual void paint(QPainter *painter, const QStyleOptionViewItem &option, 
		     const QModelIndex &index) const;
};

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So the declaration above helped me get the code to compile.

MyDelegate::MyDelegate(QWidget *parent)
  :QItemDelegate(parent)
{
  qRegisterMetaType<MyType>("MyType");
}
 
void MyDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, 
		     const QModelIndex &index) const
{
  QRect rect = option.rect;
 
  QHash<QString, QString> * t = 0;
  t = qVariantValue<QHash<QString, QString>*>(index.data(Qt::DisplayRole));
  if(t != 0)
    {
      std::cout << "T size: " << t->size() << std::endl;
    }
 
  if(option.state & QStyle::State_Selected)
    {
      painter->fillRect(rect, option.palette.highlight());
    }
  else
    {
      QItemDelegate::paint(painter, option, index);
    }
}

MyDelegate::MyDelegate(QWidget *parent)
  :QItemDelegate(parent)
{
  qRegisterMetaType<MyType>("MyType");
}

void MyDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, 
		     const QModelIndex &index) const
{
  QRect rect = option.rect;
  
  QHash<QString, QString> * t = 0;
  t = qVariantValue<QHash<QString, QString>*>(index.data(Qt::DisplayRole));
  if(t != 0)
    {
      std::cout << "T size: " << t->size() << std::endl;
    }

  if(option.state & QStyle::State_Selected)
    {
      painter->fillRect(rect, option.palette.highlight());
    }
  else
    {
      QItemDelegate::paint(painter, option, index);
    }
}

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Now something is going wrong on lines 12 and 13. Perhaps something about my data declaration below is incorrect? Of course I'm expecting that I should now have a handle on that index in the model. If I did, then I would proceed to format everything in the paint call.

QVariant CustomModel::data(const QModelIndex &index, int role)const
{
  if(!index.isValid())
    {
      return QVariant();
    }
  if(role == Qt::TextAlignmentRole)
    {
      return int(Qt::AlignRight | Qt::AlignVCenter);
    } 
  else if (role == Qt::DisplayRole)
    {
      //Returning a pointer to the QHash<QString, QString> so just 
      //cast back and then use the data in the view.
      return tableData.at(index.row());
    }
  return QVariant();  
}

QVariant CustomModel::data(const QModelIndex &index, int role)const
{
  if(!index.isValid())
    {
      return QVariant();
    }
  if(role == Qt::TextAlignmentRole)
    {
      return int(Qt::AlignRight | Qt::AlignVCenter);
    } 
  else if (role == Qt::DisplayRole)
    {
      //Returning a pointer to the QHash<QString, QString> so just 
      //cast back and then use the data in the view.
      return tableData.at(index.row());
    }
  return QVariant();  
}

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Thanks for taking your time

[kroenecker]

Ok I have it working now. I don't know that anyone else really tries to do this sort of thing, but I finally have it working. Signals and slots might cause me some problems, but I have yet to get that far in the design.

Anyway this is what I did:

1) I changed the MetaType definition so that it is no longer a pointer.
2) I moved the Q_DECLARE_METATYPE to a header file that I can include in any header files that need to access the type.
3) After a bit of difficulty I found that the only way that I can return my special typedef as a QVariant is to use QVariant::setValue(const T &value).

As a side note, I'm storing several hashtables in a QList (tableData).

#ifndef MYTYPEDEF_H
#define MYTYPEDEF_H
 
typedef QHash<QString, QString> MyType; 
Q_DECLARE_METATYPE(MyType)
 
#endif

#ifndef MYTYPEDEF_H
#define MYTYPEDEF_H

typedef QHash<QString, QString> MyType; 
Q_DECLARE_METATYPE(MyType)

#endif

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The header with the typedef.

QVariant CustomModel::data(const QModelIndex &index, int role)const
{
  if(!index.isValid())
    {
      return QVariant();
    }
  if(role == Qt::TextAlignmentRole)
    {
      return int(Qt::AlignRight | Qt::AlignVCenter);
    } 
  else if (role == Qt::DisplayRole)
    {
      //Return the QHash as a QVariant:
      const QHash<QString, QString> temp(tableData.at(index.row()));
      QVariant var;
      var.setValue(temp);
      return var;
    }
  return QVariant();  
}

QVariant CustomModel::data(const QModelIndex &index, int role)const
{
  if(!index.isValid())
    {
      return QVariant();
    }
  if(role == Qt::TextAlignmentRole)
    {
      return int(Qt::AlignRight | Qt::AlignVCenter);
    } 
  else if (role == Qt::DisplayRole)
    {
      //Return the QHash as a QVariant:
      const QHash<QString, QString> temp(tableData.at(index.row()));
      QVariant var;
      var.setValue(temp);
      return var;
    }
  return QVariant();  
}

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Returning the QVariant, which contains the QHash<QString, QString>. As I understand it, this is only possible by using Q_DECLARE_METATYPE.

[wysota]

Maybe you could have substituted the hash with a map? Then you could have used QVariantMap which fits into QVariant by default.

[kroenecker]

Wysota,

Thanks for the suggestion. I didn't notice QVariantMap.

Out of curiosity and only if you have the time, how long have you been a programmer? Are you a professional or is this just a hobby?

[wysota]

Hmm... let's see... Depends what you mean by a "programmer".
I wrote my first program probably about 16 years ago, it must have been written in BASIC for an Atari microcomputer, but it was just playing around.

I started some "real" programming probably about 12 years ago when I started learning Pascal and C. Then came other languages, my software eng. (and telecom) studies and currently I'm a PhD student at Warsaw University of Technology. I have a professional education, but obviously it's also my hobby. I wouldn't be sitting all day here otherwise regarded that nobody is paying me to do it


Some of it is mentioned here: http://www.linkedin.com/in/wysota

Oh, I started playing with Qt during my studies, about 3 years ago. Just look at my registration date at QtForum.org, it was just about then. Oh... and my first Qt application was a complete failure It was my university project from operating systems course that used threads and tried to manipulate widgets from non-gui threads. Furthermore I needed to paint rectangles, but I didn't know how to do it so I used QLabels instead. As I said - complete failure, it was hanging all the time and I didn't know why.