Re: Boxing and Unboxing ??


Peter Olcott wrote:
"Barry Kelly" <barry.j.kelly@xxxxxxxxx> wrote in message
news:2dmiq2t5l3pr1mlsmetkea1se29hfi721t@xxxxxxxxxx
Peter Olcott wrote:

"Barry Kelly" <barry.j.kelly@xxxxxxxxx> wrote:

So a member function can not add two integer members without unboxing them
first? That would sound like horrendous design.

If you carefully read what I wrote, you'll notice:

From the point of view of C#, an integer (or any other value type) is
only boxed if it's been assigned to a location of type 'object' -
whether local variable, argument or field.

You *cannot*, I repeat *CANNOT*, have two boxed integer members in C# -
the members would need to be of type *object*, not int, in order for
them to be boxed.

-- Barry

I carefully read it, yet, did not fully understand the meaning of all of the
terminology that was used. For one thing, I don't see why there is ever any need
for boxing and unboxing. I know that there is no such need in C++.

That's because in C# (and Java) you _can't_ say:

int x = 3;
int *p = &x;

because the "&" operator simply doesn't exist. You can't take the
address of an arbitrary variable.

I also know that it must somehow support GC, and that is why it is needed.

Well, more to the point, a language that supports garbage collection
can't allow one to take addresses of arbitrary memory locations, as the
garbage collector could then never determine what objects were
referenced and which weren't (because an address into the midst of an
object would then be legal).

Is it something like maintaining a chain of pointers indicating who owns what?

Well, sort of. The GC walks the stack and all static objects, looking
for references to objects on the heap. It then follows references
stored in those objects, etc, until it exhausts the network of
references. Any objects left thus unmarked are available for
collection.

Of course, it's rather more complex than that, but you get the idea. If
you allow references into the midst of objects, then it's much more
difficult to decide what is referenced and what isn't.

In C# and Java, every reference that you can directly manipulate in
code is to a valid object on the heap. That's why, if you want to treat
an int as an object (and thus have a reference to it) then the CLR has
to create an object wrapper for it and put it on the heap.

.

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