reference vs object handle

[terminator800tlgm@hotmail.com]

I don't understand the difference between ojbect handle and reference.
For example,

class Stack
{
private:
CD cd_obj; // cd object
DVD & dvd_ref;// dvd reference

Why is it that dvd_ref can ONLY be initialized in the Stack's
constructor whereas cd_obj can be initialized during the Stack
object's lifetime?

Also, why is not allowed to return by reference a locally created
object?
For example,


Vector Vector::operato r+(const Vector & b) const
{
return Vector(x+b.x, y+b.y); //Why is this allowed?

}

Vector & Vector::operato r+(const Vector & b) const
{
return Vector(x+b.x, y+b.y); //Why is this NOT allowed?

}

[Cy Edmunds]

Re: reference vs object handle

<terminator800t lgm@hotmail.com > wrote in message
news:413685a6.0 310311456.7f756 7e5@posting.goo gle.com...[color=blue]
> I don't understand the difference between ojbect handle and reference.
> For example,
>
> class Stack
> {
> private:
> CD cd_obj; // cd object
> DVD & dvd_ref;// dvd reference
>
> Why is it that dvd_ref can ONLY be initialized in the Stack's
> constructor whereas cd_obj can be initialized during the Stack
> object's lifetime?[/color]

A reference must be initialized. For instance you can't write:

int &foo;

You would have a reference to nothing in particular which is not allowed.
Actually this is a big advantage of references over pointers which can be
left uninitialized.

If dvd_ref isn't initialized in Stack's constructor it would violate this
rule.
[color=blue]
>
> Also, why is not allowed to return by reference a locally created
> object?
> For example,
>
>
> Vector Vector::operato r+(const Vector & b) const
> {
> return Vector(x+b.x, y+b.y); //Why is this allowed?[/color]

Consider the calling sequence:

Vector v = v1 + v2;

Effectively this is the same as

Vector v = Vector(v1.x + v2.x, v1.y + v2.y);

No problem here. The values of the temporary on the right hand side are
copied to the values in v. (Conceptually at least -- the compiler may
optimize that out.)
[color=blue]
>
> }
>
> Vector & Vector::operato r+(const Vector & b) const
> {
> return Vector(x+b.x, y+b.y); //Why is this NOT allowed?[/color]

Again, consider the calling sequence:

Vector &v = v1 + v2;

which is now the same as the nonsense statement

Vector &v = Vector(v1.x + v2.x, v1.y + v2.y);

Since the right hand side is a temporary object it can hardly be used to
create a valid reference.
[color=blue]
>
> }[/color]

--
Cy

http://home.rochester.rr.com/cyhome/