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Deepcopying One container [message #23574]

Mon, 02 November 2009 09:37

dolik.rce
Messages: 1791
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

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

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Re: Deepcopying One container [message #23576 is a reply to message #23574]

Mon, 02 November 2009 10:21

cbpporter
Messages: 1427
Registered: September 2007

Ultimate Contributor

Well using references works:

	{
		One<A> &c=a;
		c->DoSmthng();
	}
	{
		One<A> &c=b;
		c->DoSmthng();
	}

But since One is an encapsulation of a pointer, I believe it should not inherit <<= from DeepCopyOption, but rather do a low level copy of the pointer so that the placement new is avoided.

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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.

	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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Re: Deepcopying One container [message #23590 is a reply to message #23578]

Mon, 02 November 2009 17:57

dolik.rce
Messages: 1791
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

Thanks for good news, Mirek!

Quote:

What is your usage scenario?

I was afraid you will ask Smile

Now I have to show everyone the ugly scheme I've come up with Embarassed

I use descendants of my class PlotSymbol to provide a methods used to paint different marks in plot. Then I have a VectorMap<String,One<Plotsymbol> >. Every new symbol class is "registered" by adding into this VectorMap, so that user can be presented with a list of available symbols and choose which one to use. Everytime user changes his mind and selects new symbol, I would like just to copy the One<PlotSymbol> into local variable (and setup some additional parameters, e.g. colors or size).

So basicaly what I need is to make a copy of the underlaying PlotSymbol, without knowing what type is it. The information in manual made me believe it is possible, but then I've found that it only works when the source is picked.

I'll try the solution you suggested...

To cbporter: Thanks for your reply too. Unfortunately I need a full copy of the pointed object so I can change some additional properties without affecting the original...

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Re: Deepcopying One container [message #23596 is a reply to message #23578]

Tue, 03 November 2009 14:21

dolik.rce
Messages: 1791
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

luzr wrote on Mon, 02 November 2009 11:14

PolyDeepCopyNew works like a charm! Thank you very much Mirek, the solution was trully genial in its simplicity.

Just for future reference, the solution is something like this:

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

class A: public PolyDeepCopyNew<A>{
public:
	virtual void DoSmthng(){Cout()<<"I'm A.\n";};
	virtual A* Copy()const{return new A;}
};
class B:public A{
public: 
	virtual void DoSmthng(){Cout()<<"I'm B.\n";};
	virtual B* Copy()const{return new B;}
};

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

Honza

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Re: Deepcopying One container [message #23599 is a reply to message #23596]

Tue, 03 November 2009 19:45

mirek
Messages: 14256
Registered: November 2005

Ultimate Member

dolik.rce wrote on Tue, 03 November 2009 08:21

luzr wrote on Mon, 02 November 2009 11:14

PolyDeepCopyNew works like a charm! Thank you very much Mirek, the solution was trully genial in its simplicity.

Just for future reference, the solution is something like this:

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

class A: public PolyDeepCopyNew<A>{
public:
	virtual void DoSmthng(){Cout()<<"I'm A.\n";};
	virtual A* Copy()const{return new A;}
};
class B:public A{
public: 
	virtual void DoSmthng(){Cout()<<"I'm B.\n";};
	virtual B* Copy()const{return new B;}
};

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

Honza

IMO, the example is a very little bit misleading - Copy should deepcopy some content...

Mirek

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Re: Deepcopying One container [message #23606 is a reply to message #23599]

Wed, 04 November 2009 08:40

dolik.rce
Messages: 1791
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

luzr wrote on Tue, 03 November 2009 19:45

IMO, the example is a very little bit misleading - Copy should deepcopy some content...

Mirek

Well, the example is oversimplified. In most cases it probably should deepcopy the content - but not necessarily every time. For example in my case, all I needed was to preserve the information about type to keep the virtual methods accessible. (And I overwrite the content anyway.)

Honza

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