C++ – How to edit a QTableView

cmodel-view-controllerqt4

I am currently trying to add editing functionality to a class derived from QTableView. I have added a custom delegate that should provide editing functionality. However if I do a simple qDebug() << "FieldDelegate::createEditor()"; at the beginning of the delegate, I can see that this function never get's called. I tried to look at the examples in the Qt4 documentation and in the book "C++ Gui Programming with QT4", but all I could find were examples for delegates with QTableWidgets and not QTableViews. However in my case I need a custom data model, so I do not want to replicate any data in QTableWidgetItems.

So far I did the following:

I added the custom delegate using QAbstractItemModel::setItemDelegate()
I activated editing functions using setEditTriggers( QAbstractItemView::DoubleClicked )

When I double click on an item nothing happens in this setup. The FieldDelegate::createEditor() is not called at all.

I also tried connecting the doubleClicked() signal from the QAbstractItemView to the edit() slot of the same class, but this only gives me the message edit: editing failed whenever I double click on a cell.

So far I do not return anything in other roles than Qt::EditRole from the DatabaseModel::data() method. This will be similar to the final case, where I want to add an empty row at the bottom of the table that will be used for adding new data. Could this cause the problem?

Here is the code for the construction of the View:

DocumentChoiceView::DocumentChoiceView( DatabaseModel * model,
                                          QWidget * parent ) :
    QTableView( parent ),
    m_model( model )
  {
    setShowGrid ( false );
    setModel( m_model );
    setItemDelegate( m_model->delegate().get() );
    setEditTriggers( QAbstractItemView::DoubleClicked );
    connect( this, SIGNAL(doubleClicked(const QModelIndex&)), 
             this, SLOT(edit(const QModelIndex&)) );
    verticalHeader()->hide();
  }

Best Answer

For the model to be editable the edit flag need to be returned.

//Reimplement 
Qt::ItemFlag QAbstractItemModel::flags ( const QModelIndex & index ) const;
//and add 
Qt::ItemIsEditable 
//to the returned value

http://doc.qt.io/qt-4.8/model-view-programming.html#making-the-model-editable

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');