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C++ Question, final in a few hours

post #1 of 11
Thread Starter 
Suppose I have a parameter in c++ that is an object of an abstract class. Why is the parameter required to be a reference parameter?

^^^^^ I know that in c++( when doing object oriented) everything is a pointer to an object, so why is the above an exception? I am thinking it is because abstract classes in C++ can have purely virtual methods, so is this reference pointer used to get us the offset into the portion of the virtual method table for that object?
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post #2 of 11
I have absolutely no idea. I'm not even sure that that's true.

The one thing I will note: NEVER think of references as pointers.
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post #3 of 11
Thread Starter 
Well you access objects in C++ using pointers, with an object of an abstract class, it is a reference, why?
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post #4 of 11
@Joemaniaci, your picture is just wrong.
post #5 of 11
Thread Starter 
Quote:
Originally Posted by Abs.exe View Post
@Joemaniaci, your picture is just wrong.
Thanks.
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post #6 of 11
Because the base class is abstract? the objects located within won't be called on directly, but as a local reference to the target object of the abstract class? err bit foggy on that though, don't take my word on it lol
post #7 of 11
Thread Starter 
Quote:
Originally Posted by Asmodean View Post
Because the base class is abstract? the objects located within won't be called on directly, but as a local reference to the target object of the abstract class? err bit foggy on that though, don't take my word on it lol
We were just discussing it and we actually came to the same conclusion as you. Since we can't instantiate an object of the abstract class, we use reference parameters that reference the abstract super class. I guess it is a C++ thing where instead of going directly to the parameter of the sub-class, it has to go through the parent class. I would think there are other languages where you can refer directly to the implemented methods/variables of the abstract sub-classes.
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post #8 of 11
Quote:
Originally Posted by joemaniaci View Post
We were just discussing it and we actually came to the same conclusion as you. Since we can't instantiate an object of the abstract class, we use reference parameters that reference the abstract super class. I guess it is a C++ thing where instead of going directly to the parameter of the sub-class, it has to go through the parent class. I would think there are other languages where you can refer directly to the implemented methods/variables of the abstract sub-classes.
Yeah, thats my thinking anyway, if i remember correctly. none of that in Java ofc. C++ is pointer mania

Edit: Oh, and GL in your finals, gogo team programmers
lol
post #9 of 11
Maybe you should define what you mean by 'parameter'.

(And stop using so many pointers!)
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post #10 of 11
Abstract class variables do not need to be reference parameters in C++.

The only thing that makes a class in C++ abstract is if it has a virtual function that must be handled:
Code:
virtual void ThisFunctionMustBeHandled() = 0;
So the only requirement when using an abstract class is that any classes derived from it must have definitions for any pure virtual functions.

Quote:
I know that in c++( when doing object oriented) everything is a pointer to an object, so why is the above an exception? I am thinking it is because abstract classes in C++ can have purely virtual methods, so is this reference pointer used to get us the offset into the portion of the virtual method table for that object?
Not every member that is a class must be a pointer to an object either.

Code:
class OtherClass
{
    // Stuff
}

class TestClass
{
   private:
       OtherClass other;   // Not a pointer
}
Quote:
Originally Posted by joemaniaci View Post
Well you access objects in C++ using pointers, with an object of an abstract class, it is a reference, why?
You access objects in C++ using either pointers or references, just like any other variable.

Having a reference to an abstract class is pretty rare, usually you're going to have a reference/pointer/instance to one of the derived classes.

Code:
OtherClass other;         // An instance of a class
OtherClass& otherRef;   // A reference to an instance of a class, reference members are somewhat uncommon by the way.
OtherClass* otherPtr;   // A pointer to a class

// Accessing the members
other.Function();
otherRef.Function();
otherPtr->Function();

Edited by lordikon - 5/5/11 at 1:36pm
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