subclassed const function problem

[qtneuling]

Hi,

i subclassed a QAbstractModel and subclassed the data function
which is declared as const.
However, i need to emit a signal from there and the compiler wont let me do that because of const...
What should i do?

[mcosta]

Simply you can't emit a signal inside a const method.

This is because a signal is transformed by moc into a call to QMetaObject::activate() with "this" as "sender" argument

void MyClass::mySignal()
{
    QMetaObject::activate(this, &staticMetaObject, 0, 0);
}

void MyClass::mySignal()
{
    QMetaObject::activate(this, &staticMetaObject, 0, 0);
}

To copy to clipboard, switch view to plain text mode 

This method declare the first argument as "QObject* sender" and then it can't be const

[qtneuling]

yes i know thats what the compiler tells me.
But i NEED this signal, what should i do? create another object on which i then can call the signal from the method?

[Holy Cheater]

redeclare it as a non-const in sub-class?

[qtneuling]

so how do i do that? just deleting "const" gives me errors related to qt.

[marcel]

of course it does... inheritance doesn't work that way.
what are you trying to do? there must be another way to do it.

[Holy Cheater]

class CBase : public QObject
{
	Q_OBJECT
	public:
		CBase() : QObject(0) { someProtectedData = "something here"; };
		QString someConstFunction() const { return someProtectedData; };
	protected:
		QString someProtectedData;
};
 
class CDerived : public CBase
{
	Q_OBJECT
	public:
		QString someConstFunction() { emit someSignal(); someProtectedData = "changed"; return someProtectedData; };
	signals:
		void someSignal();
};

class CBase : public QObject
{
	Q_OBJECT
	public:
		CBase() : QObject(0) { someProtectedData = "something here"; };
		QString someConstFunction() const { return someProtectedData; };
	protected:
		QString someProtectedData;
};

class CDerived : public CBase
{
	Q_OBJECT
	public:
		QString someConstFunction() { emit someSignal(); someProtectedData = "changed"; return someProtectedData; };
	signals:
		void someSignal();
};

To copy to clipboard, switch view to plain text mode 

Constness removed in derived class. Isn't that which you are trying to do?
Another way is const_cast on "this".
Code above compiles without errors for me. Am I wrong somewhere?

qtneuling, probably you're trying to do something different.

[marcel]

Yes, but QAbstractItemModel::data is pure virtual.

[qtneuling]

i need to emit this signal because my application needs to know when an item from my model was requested and which item it was

i created a new object with a method which emits the signal, and call that from the const function. This sucks but i dont see another way.