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Re: Deepcopying One container [message #23578 is a reply to message #23574]

Mon, 02 November 2009 11:14

mirek
Messages: 14256
Registered: November 2005

Ultimate Member

dolik.rce wrote on Mon, 02 November 2009 03:37

Hi,

I've met following problem with polymorphic classes in One containers. Consider following code:

#include <Core/Core.h>
using namespace Upp;

class A{
public: 
	virtual void DoSmthng(){Cout()<<"I'm A.\n";};
};
class B:public A{
public:
	virtual void DoSmthng(){Cout()<<"I'm B.\n";};
};

CONSOLE_APP_MAIN{
	One<A> a=new A;
	One<A> b=new B;
	One<A> c;
	
	c<<=a;
	c->DoSmthng();
	
	c<<=b;
	c->DoSmthng();

The output of this is

I'm A.
I'm A.

It surprised me at first, but after looking in the implementation of operator<<=, I understood that this is to be expected (that is not a bug).

The question is: Is there some workaround to make a copy of One without loosing the information about the type it stores? I mean to make it work same way as if you do

	c<<=a;
	c->DoSmthng();
	
	c<<=b;
	c->DoSmthng();

but without a and b beeing picked. Is that even posible?

Thanks for any responses.
Regards,
Honza

Surprisingly, yes, we can provide polymorphic copies - by overloading DeepCopyNew.

That can be simplified by using PolyDeepCopyNew, using virtual
Copy method.

In reality, I have never really used polymorphic deep copy, it looks a little bit tricky to me. What is your usage scenario?

Mirek

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