Full Polymorphism ?

**mynickmynick** · 08-21-2008

please tell me if this is a correct example of polymorphism! I focused mainly on polymorphism of variables which seems to work without the use of virtual but problems??
I compiled, run and the result seems OK:

num=1
num=2.00

Code:

class ClassB
{
public:
	int num;
	virtual void setnum(){num=1;}
	virtual void printnum(){printf("num=%d\n",num);}
};

class ClassIn:public ClassB
{
public:
	float num;
	void setnum(){num=2.0;}	
	void printnum(){printf("num=%.2f\n",num);}

};

..


main()

   		ClassB B;
   		ClassIn In;
   		ClassB *CP;
   		
   		CP=&B;
   		CP->setnum();
   		CP->printnum();
   		
   		CP=&In;
   		CP->setnum();
   		CP->printnum(); 		
   		
   		getchar();

**cpjust** · 08-21-2008

Looks like polymophism to me.

**m37h0d** · 08-21-2008

just bear in mind that classb::num and classin::num are two different variables.

**mynickmynick** · 08-21-2008

Originally Posted by cpjust

Looks like polymophism to me.

but is that correct? shouldn't I put "virtual" also in front of the variable or do something different?
may be you will suggest to declare the variable protected and then access it only via member functions?
But that is not nice cause the same member function would have different return type in the two classes??

**mynickmynick** · 08-21-2008

i will try to make the problem more specific
the problem is that with my application i need to drive different industrial cameras with different APIs and specifically different Types to address grabbed images

So my case simplified would be
.. Better if put this in anew thread probably..

**laserlight** · 08-21-2008

but is that correct? shouldn't I put "virtual" also in front of the variable or do something different?

If the name hiding of ClassB::num by ClassIn::num was not intentional, perhaps what you want to do is to simply inherit the member variable (and at least declare it protected in the base class).

may be you will suggest to declare the variable protected and then access it only via member functions?

A protected member from a base class can be accessed directly by derived class member functions. If you declare the member private, as is most common for member variables, then derived class member functions would need to access such a member variable via the protected or public member functions of the base class.

But that is not nice cause the same member function would have different return type in the two classes??

You just have a toy program to learn how polymorphism works, syntactically speaking. In more realistic class design, one would ask if ClassB and ClassIn really have an is-a relationship before using public inheritance.

By the way, you might want to post an actual test program that you compiled, not some weird fragment that looks (and is) syntactically incorrect at a glance.

**CornedBee** · 08-21-2008

shouldn't I put "virtual" also in front of the variable or do something different?

Variables can never be virtual and don't take part in polymorphism. Polymorphism is about the member functions. Variables with the same name in different classes of a hierarchy have absolutely no relation.

**QuantumPete** · 08-22-2008

Just as an aside and a rule of thumb. If you have any virtual functions in a class, you should declare your own constructor and make it virtual as well.
It's not really necessary for this example, I know, but it's a good habit to get into.

QuantumPete

**matsp** · 08-22-2008

Originally Posted by QuantumPete

Just as an aside and a rule of thumb. If you have any virtual functions in a class, you should declare your own constructor and make it virtual as well.
It's not really necessary for this example, I know, but it's a good habit to get into.

QuantumPete

You mean virtual DESTRUCTOR - there is no such thing as a virtual constructor - as you can't really use the object to refer to the virtual function before it has been constructed.

--
Mats

**laserlight** · 08-22-2008

A virtual clone() function probably comes close to being a "virtual copy constructor" without actually being one, but I agree that a virtual destructor is probably what was intended.

**QuantumPete** · 08-22-2008

Woops, yes, I meant destructor. Sorry

QuantumPete

**matsp** · 08-22-2008

Going back to the original question.

If you have a base class with a member variable X, and want to REPLACE that member variable in a derived class with one that is of a different, then you are effectively hiding the original X. It would then be better to have an "empty" base-class (abstract base class) that doesn't have ANY X member, and implement two classes that derive from the base-class, one with X of type 1, and the other with X of type 2.

In the case of a camera, you may have completely different formats of the camera buffer. To implement a common functionality, you probably would have to TRANSLATE, or provide a function to read out individual pixels in a common format, since you may have one camera that packs 12 bits per pixel per plane into a buffer of 48 bits (6 bytes), and another than puts 8 bits per pixel per plane into 24 bits (3 bytes), yet another uses 8 bits per pixel, but for efficient memory handling stores it in 32 bits (4 bytes). The order of R, G, B may differ, etc, etc. In this case, you can't just have a function to fetch the buffer, but you need a function that retrieves the pixels in a uniform way.

--
Mats

**mynickmynick** · 08-23-2008

Originally Posted by matsp

Going back to the original question.

If you have a base class with a member variable X, and want to REPLACE that member variable in a derived class with one that is of a different, then you are effectively hiding the original X. It would then be better to have an "empty" base-class (abstract base class) that doesn't have ANY X member, and implement two classes that derive from the base-class, one with X of type 1, and the other with X of type 2.

In the case of a camera, you may have completely different formats of the camera buffer. To implement a common functionality, you probably would have to TRANSLATE, or provide a function to read out individual pixels in a common format, since you may have one camera that packs 12 bits per pixel per plane into a buffer of 48 bits (6 bytes), and another than puts 8 bits per pixel per plane into 24 bits (3 bytes), yet another uses 8 bits per pixel, but for efficient memory handling stores it in 32 bits (4 bytes). The order of R, G, B may differ, etc, etc. In this case, you can't just have a function to fetch the buffer, but you need a function that retrieves the pixels in a uniform way.

--
Mats

yes thank you
have a look also to the thread "Different Camera vendors driving : polymorphism "
I am actually developing that stuff in C but I would like to switch to C++ sooner or later as I will have to drive different cameras

**matsp** · 08-23-2008

Originally Posted by mynickmynick

yes thank you
have a look also to the thread "Different Camera vendors driving : polymorphism "
I am actually developing that stuff in C but I would like to switch to C++ sooner or later as I will have to drive different cameras

Of course, you don't have to switch to C++ to support multiple device in a device driver. Most Windows device drivers support multiple devices with the same driver, and they do that by using different sets of function pointers - just as an alternative solution.

Edit: http://cboard.cprogramming.com/showthread.php?t=106398, page 2, around post 20 or so, I give an example of how to do a linked list using "object oriented technique" in C.

--
Mats

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