reference vs object handle

[terminator800tlgm]

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

}

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

}

 

[Cy Edmunds]

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?

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.

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


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

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.)

}

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

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.