Casting issues...

Aaronugio · 07-20-2009, 02:08 PM

Ok, i think I've got it now, i adjusted the "new Animal" to being a "new Dog" and added a call to the destructor and it works now.

Thanks again so much for being patient with me, I know I can be pretty hard to explain things to.

cpjust · 07-20-2009, 05:19 PM

BTW, in real world code, you'd want to check the return value of dynamic_cast (when used with pointers) to see if it's NULL, which means the pointer isn't the right type; or put it in a try/catch block around a dynamic_cast of a reference and catch a bad_cast exception, which also means the reference isn't the right type.

Code:

Animal* a = new Cat;
Dog* p = dynamic_cast<Dog*>( a );

if ( p == NULL )
{
    cout << "Oops, I guess 'a' isn't really a Dog!" << endl;
}

Animal& b = *a;

try
{
    Dog& r = dynamic_cast<Dog&>( b );
}
catch ( std::bad_cast& e )
{
    cout << "Oops, I guess 'b' isn't really a Dog!" << endl;
}

hk_mp5kpdw · 07-21-2009, 05:43 AM

Your destructor's should be virtual.

Aaronugio · 07-22-2009, 10:35 AM

Thanks guys for all the help,
I have one last question on the subject,
let's say i have Dog, object, but want it to behave like a Cat object, assuming they're both on the same hierarchy level, would it be more efficient to use casting at all, or just destroy the Dog, and recreate the object as a Cat, and go with it from there?

**laserlight** · 07-22-2009, 10:41 AM

Quote:

Originally Posted by Aaronugio

let's say i have Dog, object, but want it to behave like a Cat object, assuming they're both on the same hierarchy level, would it be more efficient to use casting at all, or just destroy the Dog, and recreate the object as a Cat, and go with it from there?

It is not so much a matter of efficiency as a matter of correctness: you have a dog, but you want a cat, so if you can only have one animal, then you must get rid of the dog and then get the cat. You cannot turn the dog into a cat.

Aaronugio · 07-22-2009, 10:58 AM

well it's not so much that i want the Dog to BE a cat so much as i want it to be able to perform a couple of Cat functions

or would it be more effective to just write the two or so Cat functions as protected functions within the parent Mammal Class

**laserlight** · 07-22-2009, 11:01 AM

Quote:

Originally Posted by Aaronugio

well it's not so much that i want the Dog to BE a cat so much as i want it to be able to perform a couple of Cat functions

Then you need to rethink your design.

Elysia · 07-22-2009, 11:02 AM

Why would a Dog need Cat functions? If it needs that, then the design should be flawed.
You should try to keep functionality that both classes need in a common base class, but it should also be something one can expect from that class. Like a Dog not meowing.

cpjust · 07-22-2009, 08:17 PM

So basically, you want a Dog that's afraid of water and says "Meow"?

Sebastiani · 07-22-2009, 08:54 PM

>> So basically, you want a Dog that's afraid of water and says "Meow"?

Sort of like how Mr. Ed can talk like a human, neigh?

07-20-2009, 02:08 PM	#16
Aaronugio Registered User Join Date: Jul 2009 Posts: 14	Ok, i think I've got it now, i adjusted the "new Animal" to being a "new Dog" and added a call to the destructor and it works now. Thanks again so much for being patient with me, I know I can be pretty hard to explain things to.
Aaronugio is offline

07-20-2009, 05:19 PM	#17
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	BTW, in real world code, you'd want to check the return value of dynamic_cast (when used with pointers) to see if it's NULL, which means the pointer isn't the right type; or put it in a try/catch block around a dynamic_cast of a reference and catch a bad_cast exception, which also means the reference isn't the right type. Code: Animal* a = new Cat; Dog* p = dynamic_cast<Dog>( a ); if ( p == NULL ) { cout << "Oops, I guess 'a' isn't really a Dog!" << endl; } Animal& b = a; try { Dog& r = dynamic_cast<Dog&>( b ); } catch ( std::bad_cast& e ) { cout << "Oops, I guess 'b' isn't really a Dog!" << endl; } __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

07-21-2009, 05:43 AM	#18
hk_mp5kpdw Registered User Join Date: Jan 2002 Location: Northern Virginia/Washington DC Metropolitan Area Posts: 2,787	Your destructor's should be virtual. __________________ On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question. --Charles Babbage, 1792-1871 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
hk_mp5kpdw is offline

07-22-2009, 10:35 AM	#19
Aaronugio Registered User Join Date: Jul 2009 Posts: 14	Thanks guys for all the help, I have one last question on the subject, let's say i have Dog, object, but want it to behave like a Cat object, assuming they're both on the same hierarchy level, would it be more efficient to use casting at all, or just destroy the Dog, and recreate the object as a Cat, and go with it from there?
Aaronugio is offline

07-22-2009, 10:58 AM	#21
Aaronugio Registered User Join Date: Jul 2009 Posts: 14	well it's not so much that i want the Dog to BE a cat so much as i want it to be able to perform a couple of Cat functions or would it be more effective to just write the two or so Cat functions as protected functions within the parent Mammal Class
Aaronugio is offline

07-22-2009, 08:17 PM	#24
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	So basically, you want a Dog that's afraid of water and says "Meow"? __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

07-22-2009, 08:54 PM	#25
Sebastiani Guest Join Date: Aug 2001 Posts: 4,923	>> So basically, you want a Dog that's afraid of water and says "Meow"? Sort of like how Mr. Ed can talk like a human, neigh?
Sebastiani is offline

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