inheritence and virtual

[Cruz]

Hello!

Admittedly, I'm not the most experienced C++ programmer and this is the first time I'm trying to use inheritance and virtual functions. It doesn't work like it should and I can't figure out why. Here is the juice.

class PathPiece
{
public:
 
	virtual void pos()
	{
		qDebug() << "Called base";
	}
 
	void draw(QPainter* painter)
	{
		pos();
	}
};
 
 
class LinePiece : public PathPiece
{
public:
 
	void  pos()
	{
		qDebug() << "Called line";
	}
};

class PathPiece
{
public:

	virtual void pos()
	{
		qDebug() << "Called base";
	}

	void draw(QPainter* painter)
	{
		pos();
	}
};


class LinePiece : public PathPiece
{
public:

	void  pos()
	{
		qDebug() << "Called line";
	}
};

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LinePiece is a kinf of PathPiece and it's supposed to override the pos() function, while the draw() function remains central in PathPiece. If I do

LinePiece lp;
lp.draw(painter);

LinePiece lp;
lp.draw(painter);

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Then everything works. However, if I do

QList<PathPiece> list;
list << LinePiece();
list[0].draw();

QList<PathPiece> list;
list << LinePiece();
list[0].draw();

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then it doesn't. The pos() function of the base class is called. Isn't virtual supposed to be for cases exactly like this?

[Zlatomir]

You need to use pointers to achieve that, something like this:

QList<PathPiece*> list;

QList<PathPiece*> list;

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you are truncating your values if you don't.

So: Base obj = Derived objD; //isn't valid it performs a cast and you will have a Base object
You need Base* obj = Derived *objD; // and this is ok, virtual mechanism works with this

[Cruz]

Thank you for the clarification!

[Zlatomir]

Your welcome, careful the last two line isn't c++ code, i said it in c++ style, just to make an example.

And for your Base class write a destructor and declare it virtual (so the objects get destroyed properly no mater what object the Base pointer points to)