Forward declaration in Qt project - c++

Question

I'm trying to use Qt with C++. I'd use QT for programming in Python before.
my simple test doesn't work. This is my tour.h file:
#ifndef TOUR_H
#define TOUR_H
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QTableView>
class TourTable;
class Tour : public QMainWindow
{
Q_OBJECT
public:
Tour();
/*protected:
void closeEvent(QCloseEvent *event);
private slots:
void newCompetitor();
void remCompetitor();
void finalReport();
void openPage();
void savePage();
void reloadTitle();*/
private:
TourTable _table;
};
class QStandardItem;
class QStandardItemModel;
class TourTable : public QTableView
{
Q_OBJECT
public:
TourTable();
/* bool isChanged();
QString windowName();
void finalReport();
void newCompetitor();
void remCompetitor();
bool savePage();
void setChanged(bool value);
void openPage();
protected:
void itemChanged(QStandardItem item);
private:*/
// bool _secondBetter(p1, p2);
Tour _parent;
// QStandardItemModel _model;
// bool _saved;
// bool _changed;
};
#endif
I'd commented almost everything in this code to isolate the problem, but I still don't know what is causing this. Is my first trying in C++.
The error message is:
tour.h:28:12: error: field ‘_table’ has incomplete type ‘TourTable’
TourTable _table;
^~~~~~
tour.h:7:7: note: forward declaration of ‘class TourTable’
class TourTable;
Can someone help me to solve that?

Answer 1

Forward declaration in C++ allows to reference a type before its definition. Problem here is that in your code:
class TourTable;
class Tour : public QMainWindow
{
// ...
TourTable _table;
};
You're not only referencing the type TourTable, you're instantiating it with TourTable _table;. This requires a complete definition of TourTable.
A solution could be to define TourTable before Tour, as follows:
class TourTable : public QTableView
{
// ...
Tour _parent;
}
class Tour : public QMainWindow
{
// ...
TourTable _table;
};
But that just moves the problem since you're instantiating Tour in TourTable.
Depending on the complete design, the solution may be to use pointers. Something like that:
class TourTable;
class Tour : public QMainWindow
{
Tour();
// forward declaration of the constructor,
// see below for the definition
TourTable* _table;
// no complete definition of TourTable at this stage,
// only a forward declaration:
// we can only declare a pointer
};
class TourTable : public QTableView
{
TourTable(Tour* parent):
QTableView(parent),
_parent(parent)
{
}
Tour* _parent;
}
Tour::Tour() // definition of Tour's constructor
{
_table = new TourTable(this);
// TourTable has been defined : we can instantiate it here
}

Answer 2

To be clear, this error is coming from your compiler -- not from qmake -- and is a limitation of the language. But it can be easily overcome by creating a class that defines a parent/child behavior and making any classes where you want that behavior to inherit from the base class, as Qt's QObject does.
Both QMainWindow and QTableView inherit from QObject, so if you make use of QObject's parent/child system this design may be redundant. To find the parent from the child call parent(), and to find the children from the parent you can call children().