Implicit dereferences rational question

Implicit dereferences rational question

[Victor Porton]

Why implicit dereferences use only access discriminants, not arbitrary 
access fields?

-- 
Victor Porton - http://portonvictor.org

Re: Implicit dereferences rational question

[Robert A Duff]

Victor Porton <porton@narod.ru> writes:

> Why implicit dereferences use only access discriminants, not arbitrary 
> access fields?

It helps prevent dangling pointers.

- Bob

Re: Implicit dereferences rational question

[Victor Porton]

Robert A Duff wrote:
> Victor Porton <porton@narod.ru> writes:
> 
>> Why implicit dereferences use only access discriminants, not arbitrary
>> access fields?
> 
> It helps prevent dangling pointers.

In which way limiting to discriminants may prevent dangling pointers?

-- 
Victor Porton - http://portonvictor.org

Re: Implicit dereferences rational question

[Randy Brukardt]

"Victor Porton" <porton@narod.ru> wrote in message 
news:llb1aa$gth$1@speranza.aioe.org...
> Robert A Duff wrote:
>> Victor Porton <porton@narod.ru> writes:
>>
>>> Why implicit dereferences use only access discriminants, not arbitrary
>>> access fields?
>>
>> It helps prevent dangling pointers.
>
> In which way limiting to discriminants may prevent dangling pointers?

Discriminants have a lengthy set of accessibility rules, which turn out to 
be exactly what's needed for limiting dangling pointers. In particular, it's 
almost never the case that one can make a copy of an access discriminant, 
while that's easy for a "arbitrary access field".

                                        Randy.

Re: Implicit dereferences rational question

[Robert A Duff]

"Randy Brukardt" <randy@rrsoftware.com> writes:

> "Victor Porton" <porton@narod.ru> wrote in message 
> news:llb1aa$gth$1@speranza.aioe.org...
>> Robert A Duff wrote:
>>> Victor Porton <porton@narod.ru> writes:
>>>
>>>> Why implicit dereferences use only access discriminants, not arbitrary
>>>> access fields?
>>>
>>> It helps prevent dangling pointers.
>>
>> In which way limiting to discriminants may prevent dangling pointers?
>
> Discriminants have a lengthy set of accessibility rules, which turn out to 
> be exactly what's needed for limiting dangling pointers. In particular, it's 
> almost never the case that one can make a copy of an access discriminant, 
> while that's easy for a "arbitrary access field".

Right; thanks, Randy.

Victor, I suggest you try to create dangling pointers via those
operations that return things with access discriminants.
When you see the error messages from the compiler, you
will likely understand why it's hard to create dangling
pointers by accident.

An alternative is to read and understand the accessibility
rules.  That's quite hard work!

And note that a discriminant of a named access type is not an
"access discriminant"; the access type has to be anonymous for
the special accessibility rules to kick in.

- Bob

Re: Implicit dereferences rational question

[Dmitry A. Kazakov]

On Mon, 19 May 2014 16:51:56 -0500, Randy Brukardt wrote:

> "Victor Porton" <porton@narod.ru> wrote in message 
> news:llb1aa$gth$1@speranza.aioe.org...
>> Robert A Duff wrote:
>>> Victor Porton <porton@narod.ru> writes:
>>>
>>>> Why implicit dereferences use only access discriminants, not arbitrary
>>>> access fields?
>>>
>>> It helps prevent dangling pointers.
>>
>> In which way limiting to discriminants may prevent dangling pointers?
> 
> Discriminants have a lengthy set of accessibility rules, which turn out to 
> be exactly what's needed for limiting dangling pointers. In particular, it's 
> almost never the case that one can make a copy of an access discriminant, 
> while that's easy for a "arbitrary access field".

Except that dereference, be it implicit or explicit, does not create
pointers. Obviously, it eliminates a pointer, as the name suggests.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Implicit dereferences rational question

[Randy Brukardt]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1pkkrdwu7lh6y.7cjyncdfw33e.dlg@40tude.net...
> On Mon, 19 May 2014 16:51:56 -0500, Randy Brukardt wrote:
>
>> "Victor Porton" <porton@narod.ru> wrote in message
>> news:llb1aa$gth$1@speranza.aioe.org...
>>> Robert A Duff wrote:
>>>> Victor Porton <porton@narod.ru> writes:
>>>>
>>>>> Why implicit dereferences use only access discriminants, not arbitrary
>>>>> access fields?
>>>>
>>>> It helps prevent dangling pointers.
>>>
>>> In which way limiting to discriminants may prevent dangling pointers?
>>
>> Discriminants have a lengthy set of accessibility rules, which turn out 
>> to
>> be exactly what's needed for limiting dangling pointers. In particular, 
>> it's
>> almost never the case that one can make a copy of an access discriminant,
>> while that's easy for a "arbitrary access field".
>
> Except that dereference, be it implicit or explicit, does not create
> pointers. Obviously, it eliminates a pointer, as the name suggests.

I suppose, but you have to have a pointer or something very similar to a 
pointer if you want to be able to write to it. And that's the key feature 
here: that one can use these as variables as well as constants. It didn't 
make much sense to invent a new, almost pointer construct just for this 
purpose when we already had one with the correct semantics.

                                Randy. 

Re: Implicit dereferences rational question

[Dmitry A. Kazakov]

On Tue, 20 May 2014 16:26:08 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:1pkkrdwu7lh6y.7cjyncdfw33e.dlg@40tude.net...
>> On Mon, 19 May 2014 16:51:56 -0500, Randy Brukardt wrote:
>>
>>> "Victor Porton" <porton@narod.ru> wrote in message
>>> news:llb1aa$gth$1@speranza.aioe.org...
>>>> Robert A Duff wrote:
>>>>> Victor Porton <porton@narod.ru> writes:
>>>>>
>>>>>> Why implicit dereferences use only access discriminants, not arbitrary
>>>>>> access fields?
>>>>>
>>>>> It helps prevent dangling pointers.
>>>>
>>>> In which way limiting to discriminants may prevent dangling pointers?
>>>
>>> Discriminants have a lengthy set of accessibility rules, which turn out 
>>> to
>>> be exactly what's needed for limiting dangling pointers. In particular, 
>>> it's
>>> almost never the case that one can make a copy of an access discriminant,
>>> while that's easy for a "arbitrary access field".
>>
>> Except that dereference, be it implicit or explicit, does not create
>> pointers. Obviously, it eliminates a pointer, as the name suggests.
> 
> I suppose, but you have to have a pointer or something very similar to a 
> pointer if you want to be able to write to it.

Actually not. Most pointers and pointer-like objects are initialized once
and never change.

But this still has nothing to do with implicit dereference, or more
precisely with delegation of operations through composition with X.all.

> And that's the key feature 
> here: that one can use these as variables as well as constants. It didn't 
> make much sense to invent a new, almost pointer construct just for this 
> purpose when we already had one with the correct semantics.

Of course it does. So many projects and practically all bindings require
smart pointers, handles and other user-defined referential objects in some
or other form.

It is about language design principles. Programmers want to control the
semantics of referential types. Ada tried to enforce a certain semantics on
them in the ugly form of incomprehensible accessibility rules.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de