Teach me CS stuff for interviews

[Reincarnate]

Re: Teach me CS stuff for interviews

I'm not sure what virtual/pure virtual really means so maybe that is impeding my understanding of this.

In my mind, the whole point of inheritance was to have some sort of logical structure of abstraction, where the parent (or whatever you call it, the class you inherit) class has methods/data attributes that a bunch of subclasses have in common. Like a Piece class in Chess or something where Rook, Queen, King etc all inherit from or something (just an example), as they all have their separate handling methods but they're all ultimately pieces that follow the same general rules.

So when we're talking about this polymorphism concept, I keep thinking "Why the heck is this even the case?" If I want to call a cat's make_sound, I would declare a cat and call make_sound. If I want to call some animal's make_sound, I would declare an animal object and call make_sound.

Unless this idea is to have a cat object that can "choose" somehow to all either its local make_sound or its parent class' make_sound? Am I anywhere close on any of this? What's the point of this polymorphism concept?

[FissionMailed1]

Re: Teach me CS stuff for interviews

Well, if you have a "piece", but you want to call move() on that piece, how exactly does that piece move? You wouldn't know how to "move" unless you implemented a specific way to move depending on which type of piece it is (the subclass).

Your last questions are correct. You can choose which make_sound() is called.

Another example of polymorphism, using a function:

Piece* some_piece = new Rook()

move(some_piece*);

void move (piece* piece_to_move) {
// do some stuff to move in a generic way, disregarding which type of piece it is
}

This is all assuming that move() is either virtual or pure virtual.

[Reincarnate]

Re: Teach me CS stuff for interviews

yeah I am not getting this, lol.

I'm still not seeing what the differentiating notion is here -- what this accomplishes that I can't already do by just localizing the move/make_sound functions to what they logically apply to.

All animals make sound, and all cats/dogs make sounds, too. But to me, this just implies that I could have make_sound in the animal class and have the sound variable/file/whatever localized to the cat and dog classes. That way when I call make_sound() on a cat object, it takes the sound variable in cat and plays it via the animal's make_sound() function. Or something to that logical effect.

[FissionMailed1]

Re: Teach me CS stuff for interviews

That way when I call make_sound() on an animal object, it takes the sound variable in cat and plays it via the cat's make_sound() function. Or something to that logical effect.

This is the correct statement, but I think you are almost understanding it! It would call cat's make_sound() if the animal object is a pointer to a cat object.

EDIT: I think it would make more sense if we just assume that make_sound() is pure virtual in animal, so animal's make_sound() can never be directly called. It can only be called through its derived class.

[Reincarnate]

Re: Teach me CS stuff for interviews

So I could say "Animal* kitty = new Cat()" but what I am saying is that why couldn't I just do "Cat kitty = new Cat()" where Cat inherits Animal, and then when I call kitty.make_sound(), it calls the Animal's make_sound() function using the data attributes of the Cat() object?

[FissionMailed1]

Re: Teach me CS stuff for interviews

It will call Animal's make_sound() function, yes. However, if Animal's make_sound() function is pure virtual, it will look at what type of object Animal is pointing to (in this case, a cat) and call that implementation of make_sound() instead. It will just the data attributes it inherited from animal (if there is any) and the data attributes from Cat when executing make_sound().

Oh, I should also mention, pure virtual is declared as follow:

virtual void foo() = 0;

EDIT: Sorry, I misread your post. my post is assuming that you are executing this:

Animal* kitty = new Cat();
kitty->make_sound();

EDIT 2: I need to go eat some food or something, not thinking straight. If you do something like this:

Cat* kitty = new Cat();
kitty->make_sound();

This would also be valid code as well, and will perform exactly the same as the code snippet in my first edit.

[Reincarnate]

Re: Teach me CS stuff for interviews

Can't I just declare Cat kitty = new Cat()?

[FissionMailed1]

Re: Teach me CS stuff for interviews

You could do Cat kitty = Cat();. You cannot declare kitty on the stack and then use new() because new allocates memory on the heap, so kitty would have to be a pointer to a Cat if you wanted to use new(). If you use Cat kitty = Cat();, you would write kitty.make_sound() rather then kitty->make_sound() because the -> operator is for object pointers.

[UserNameGoesHere]

Re: Teach me CS stuff for interviews

Okay Reincarnate, I made some code for you :-P

#include <iostream>

class Animal {
public:
Animal();
~Animal();
/*virtual*/ void make_sound();
};

class Cat : public Animal {
public:
Cat();
~Cat();
void make_sound();
};

Animal::Animal(){}
Animal::~Animal(){}

void Animal::make_sound(){
	std::cout << "ROAAAAARRRR" << std::endl;
}

Cat::Cat(){}
Cat::~Cat(){}

void Cat::make_sound(){
	std::cout << "Meow" << std::endl;
}

int main(int argc, char* argv[]){
	std::cout << "Testing animals" << std::endl;
	
	Animal* animal = new Animal();
	Cat* cat = new Cat();
	Animal* catanimal = dynamic_cast<Animal*>(new Cat());

	std::cout << "animal says "; animal->make_sound();
	std::cout << "cat says "; cat->make_sound();
	std::cout << "catanimal says "; catanimal->make_sound();

	delete animal;
	delete cat;
	delete catanimal;

	return 0;
}

If you declare without the virtual keyword (as is above, where it is commented-out) you get as output

Testing animals
animal says ROAAAAARRRR
cat says Meow
catanimal says ROAAAAARRRR

But if you do declare the virtual keyword (remove the commenting) you get as output

Testing animals
animal says ROAAAAARRRR
cat says Meow
catanimal says Meow

Or, basically, yes the virtual keyword does matter a whole lot, and FissionMailed is right.

[Reincarnate]

Re: Teach me CS stuff for interviews

[UserNameGoesHere]

Re: Teach me CS stuff for interviews

Also, I think a lot of people miss the point of implementing things like linked lists, hash tables, etc...

Yes, they've been done before. Yes, you could find a code somewhere and copy/paste. But that's not the point. The point isn't to implement those specific things with known solutions, but to see if you can problem solve just in general, to see if you could possibly solve problems which don't have known solutions, which are interesting.

If you can't implement the simple stuff then how can you be expected to implement anything more complex?

[FissionMailed1]

Re: Teach me CS stuff for interviews

LMFAO, that gif. I've been where you are before, and I understand your frustration. It took me a while to understand polymorphism as well, especially in the context of C++. I can give you java examples if you want and it might be easier to understand, but the reason I gave C++ examples is so the details of what exactly is going on aren't hidden by the programming language.

[Reincarnate]

Re: Teach me CS stuff for interviews

UNGH: If anyone made that argument to me in person, I'd just tell them, "Go beat PE 344, 361, and 415 -- and see if you can beat my runtime, code length, and clarity"

FM: Nah I've only used C++ IME, no Java yet although I should learn that too.

[FissionMailed1]

Re: Teach me CS stuff for interviews

Java isn't drastically different from C++ at a rudimentary level, so I don't think you would have that much of an issue learning it very quickly. The main difference is that it doesn't have as many low level operations as C++ does, and is a bit more verbose; that's pretty much it though. It is very object-oriented by design so it has interfaces and abstract classes built into the language, among other things.

[Reincarnate]

Re: Teach me CS stuff for interviews

yeah, according to the video playlist posted earlier, Java programming doesn't revolve much around pointer management but instead Java has references which are automatically handled by the garbage collector when things go out of scope. A lot of the syntax between Java and C++ is the same (or similar), too