#include <iostream>

using namespace std;

/* Component (interface) */
class Widget {

public:
virtual void draw() = 0;
virtual ~Widget() {}

};

/* ConcreteComponent */
class TextField : public Widget {

private:
int width, height;

public:
TextField( int w, int h ){
width = w;
height = h;
}
void draw() {
cout << "TextField: " << width << ", " << height << '\n';
}

};

/* Decorator (interface) */
class Decorator : public Widget {

private:
Widget* wid; // reference to Widget
public:
Decorator( Widget* w ) {
wid = w;
}
void draw() {
wid->draw();
}

~Decorator() {
delete wid;
}

};

/* ConcreteDecoratorA */
class BorderDecorator : public Decorator {

public:
BorderDecorator( Widget* w ) : Decorator( w ) { }
void draw() {
Decorator::draw();
cout << " BorderDecorator" << '\n';
}

};

/* ConcreteDecoratorB */
class ScrollDecorator : public Decorator {
public:
ScrollDecorator( Widget* w ) : Decorator( w ) { }
void draw() {
Decorator::draw();
cout << " ScrollDecorator" << '\n';
}

};

int main( void ) {

Widget* aWidget = new BorderDecorator( new ScrollDecorator( new
TextField( 80, 24 )));

aWidget->draw();

delete aWidget;

}

+++++++++++++++++++++++++++++++++++++++++++
How does interface property changes here ?

it will call draw( ) function in following order
1. TextField
2. ScrollDecorator
3. BorderDecorator

Is it Following RAII( Resource Acquisition is Order of
initialization ) principle of C++ ?

How are virtual function helping it ?

++++++++++++++++++++++++++++++++++++++++++

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Pallav singh wrote:
[code stripped to the minimum]
> class Widget {
> virtual void draw() = 0;
> };
> class TextField : public Widget { ... };
>
> class Decorator : public Widget {
> public:
> Decorator( Widget* w ): wid(w) {}
> void draw() {
> wid->draw();
> }
> };
>
> class BorderDecorator : public Decorator { ... };
>
> int main( void ) {
> Widget* aWidget = new BorderDecorator( new TextField( 80, 24 ));
> aWidget->draw();
> }

> How does interface property changes here ?

Huh? I'm not sure what you mean here. What is the 'interface property'? And
once you define that and 'here', it should be trivial to define whether it
changes.

> Is it Following RAII( Resource Acquisition is Order of
> initialization ) principle of C++ ?

Sorry, but you got that abbreviation wrong, there is no 'order' in RAII.
Anyway, typically RAII is used in the meaning that the lifetime of a
resource or some other operation is bound to the scope of an object, using
constructor and destructor. Which constructors and destructors of automatic
objects are called in your example?

> How are virtual function helping it ?

Just implement 'it' (whatever exactly 'it' is...) without them as an example
and you will have hands-on experience.

Note: I've been intentionally vague in some answers because your questions
smell like homework, and while I don't mind helping with homework, you
don't seem to show an effort in solving it yourself and I'm not supporting
this perceived lazy attitude.

Uli


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