Newbie question about virtual functions

[Jim]

Hi, can someone explain to me if virtual functions need to be invoked
by a pointer to the object (not just a normal object) in order to
facilitate polymorphism?

So, in this example:

class BaseClass
{
...
virtual vfunc();
...
}

class DervicedClass
{
...
vfunc();
...
}

BaseClass b;
DerviedClass d;
b = d;
b.vfunc();

Will DerivedClass's vfunc be called?

Thanks in advance.

 

[Victor Bazarov]

Hi, can someone explain to me if virtual functions need to be invoked
by a pointer to the object (not just a normal object) in order to
facilitate polymorphism?

It doesn't.

So, in this example:

class BaseClass
{
...
virtual vfunc();

virtual void vfunc();

...
}

;

class DervicedClass

class DerivedClass : public BaseClass

{
...
vfunc();

void vfunc();

...
}

;

BaseClass b;
DerviedClass d;
b = d;
b.vfunc();

Will DerivedClass's vfunc be called?

Of course not. 'b' has the type 'BaseClass'.

V

 

[Gavin Deane]

Hi, can someone explain to me if virtual functions need to be invoked
by a pointer to the object (not just a normal object) in order to
facilitate polymorphism?

A pointer or reference, yes.

So, in this example:

class BaseClass
{
...
virtual vfunc();
...
}

class DervicedClass
{
...
vfunc();
...
}

BaseClass b;
DerviedClass d;
b = d;
b.vfunc();

Will DerivedClass's vfunc be called?

No. b is of type BaseClass. When you did

b = d;

you did what is called slicing. You sliced off the BaseClass part of
the DerivedClass object d and assigned it to the BaseClass object b.
This is unlikely to be what you want and should usually be avoided.

A BaseClass object can only ever be a BaseClass object. It can not be a
DerivedClass object "under the hood".

An object of type pointer-to-BaseClass
BaseClass* pb
can actually point to a on object of type DerivedClass, and similarly
for a reference
BaseClass& rb

Gavin Deane

 

[Marcus Kwok]

No. b is of type BaseClass. When you did

b = d;

you did what is called slicing. You sliced off the BaseClass part of
the DerivedClass object d and assigned it to the BaseClass object b.

I always interpreted it as the other way around: The DerivedClass part
of d gets sliced off, leaving only the BaseClass part, which gets
assigned to b.

 

[Tom Widmer]

Hi, can someone explain to me if virtual functions need to be invoked
by a pointer to the object (not just a normal object) in order to
facilitate polymorphism?

So, in this example:

class BaseClass
{
...
virtual vfunc();
...
}

class DervicedClass
{
...
vfunc();
...
}

BaseClass b;
DerviedClass d;
b = d;
b.vfunc();

Will DerivedClass's vfunc be called?

No. b above is-a BaseClass (you can't change the type of an object once
it has been created). When you assign the d object to it, BaseClass&
operator=(BaseClass const&) is called - e.g. only the BaseClass part of
d is assigned. This is called 'slicing'.

For polymorphism, you need pointers or references.

Tom

 

[Victor Bazarov]

I always interpreted it as the other way around: The DerivedClass
part of d gets sliced off, leaving only the BaseClass part, which gets
assigned to b.

There is no "DerivedClass part". DerivedClass instance includes BaseClass
as its part. IOW, "DerivedClass" is _larger_ than "BaseClass", so one
can't use the expression "DerivedClass part" when talking of the instance.
I know, I know, you probably want it to mean "The part that DerivedClass
adds to BaseClass", but it just comes out wrong.

V

 

[Marcus Kwok]

There is no "DerivedClass part". DerivedClass instance includes BaseClass
as its part. IOW, "DerivedClass" is _larger_ than "BaseClass", so one
can't use the expression "DerivedClass part" when talking of the instance.
I know, I know, you probably want it to mean "The part that DerivedClass
adds to BaseClass", but it just comes out wrong.

Yes. How about, "The DerivedClass-only part"?

 

[Jim]

Okay, this make more sense now.

This may be a stupid question, but why doesn't slicing occur with
pointers or references? Is it because the pointer just contains a
memory address and not the actual object?

Thanks for the responses.

 

[Tom Widmer]

Okay, this make more sense now.

This may be a stupid question, but why doesn't slicing occur with
pointers or references? Is it because the pointer just contains a
memory address and not the actual object?

Slicing only occurs when you make a copy of an object. If you are just
assigning its address to a pointer, or binding it to a reference, no
copying of the object occurs.

e.g.

B b;
A a(b); //b is "sliced" and it's A part is copied into a.


A* ap = &b; //no copying of b
A& ar = b; //no copying of b

Tom

 

[=?ISO-8859-1?Q?Martin_J=F8rgensen?=]

Okay, this make more sense now.

This may be a stupid question, but why doesn't slicing occur with
pointers or references? Is it because the pointer just contains a
memory address and not the actual object?

That's how I think of it...


Best regards / Med venlig hilsen
Martin Jørgensen