C ++类的常量参数

Question

I'm implementing custom TableModel ( MyTableModel class) in Qt, based on QAbstractTableModel to display the data stored in an object of MyDataStorage class. It requires defining rowCount() and columnCount() functions for MyTableModel .

In my case I have a class MyDataStorage that contains a QList<MyData> dataList . MyData class consists of a five member variables (one int , one QDate and three QString s). MyTableModel::rowCount() returns simply the length of MyDataStorage 's dataList . MyTableModel::columnCount() must return the number of members of MyData that are to be shown in the table, so first I defined the public method MyData::getParameterCount() {return 5;} , so that I could use it as shown below:

class MyTableModel : public QAbstractTableModel
{
    Q_OBJECT
    MyDataStorage& im;
public:
    explicit InfoTableModel(MyDataStorage& m, QObject *parent = 0);
    int rowCount();
    inline int columnCount() {return m.getLast().getParameterCount()};
    QVariant data(const QModelIndex &index, int role) const;
    QVariant headerData(int section, Qt::Orientation orientation, int role) const;
//...
}

I feel it is not the best solution as the list can be empty. Other possibilites are to define:

• equivalent function in MyDataStorage ,
• preprocessor constant,
• static member of MyData ,
• simple inline int columnCount() {return 5;} as MyTableModel has to be changed with every change of the underlying data model (it's the most obvious solution, but I'm scared if this is correct)

but I don't feel any of them is the proper solution, as the number of MyData members will change in the future.

What are your propositions?

Answer 1

If you do not have an element in your storage, then your row count is zero, as is your column count. Therefore you should just check this:

inline int columnCount() { return (m.isEmpty() ? 0 : m.getLast().getParameterCount()) };

Answer 2

You can declare MyData::getParameterCount() as static member and then refer to it like this:

inline int columnCount() const {return MyData::getParameterCount();}

In that case you don't need any MyData objects.