Who can provide a link to an explanation about why in D has taken to distinguish between classes and structures?

(Sorry for my bad English)

On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
> Who can provide a link to an explanation about why in D has taken to distinguish between classes and structures?
>
> (Sorry for my bad English)
>

The main justification is eliminating the slicing problem. http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html

D solves it by making it impossible to have a class instance as a value type.

--bb

Bill Baxter пишет:
> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
>> Who can provide a link to an explanation about why in D has taken to
>> distinguish between classes and structures?
>>
>> (Sorry for my bad English)
>>
> 
> The main justification is eliminating the slicing problem.
> http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html
> 
> D solves it by making it impossible to have a class instance as a value type.
> 


Why not disallow the casting for structs, leaving the possibility of up casting for the links and pointers to the structure?

Weed пишет:
> Bill Baxter пишет:
>> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
>>> Who can provide a link to an explanation about why in D has taken to
>>> distinguish between classes and structures?
>>>
>>> (Sorry for my bad English)
>>>
>>
>> The main justification is eliminating the slicing problem.
>> http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html 
>>
>>
>> D solves it by making it impossible to have a class instance as a value type.
>>
> 
> 
> Why not disallow the casting for structs, leaving the possibility of up casting for the links and pointers to the structure?

What is the best place for such questions?

2008/12/15 Weed <resume755@mail.ru>:
> Bill Baxter пишет:
>>
>> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
>>>
>>> Who can provide a link to an explanation about why in D has taken to distinguish between classes and structures?
>>>
>>> (Sorry for my bad English)
>>>
>>
>> The main justification is eliminating the slicing problem.
>>
>> http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html
>>
>> D solves it by making it impossible to have a class instance as a value type.
>>
>
>
> Why not disallow the casting for structs, leaving the possibility of up casting for the links and pointers to the structure?

I have to confess I don't really understand this question.  Can you rephrase or give an example?

--bb

2008/12/15 Weed <resume755@mail.ru>:
> Weed пишет:
>>
>> Bill Baxter пишет:
>>>
>>> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
>>>>
>>>> Who can provide a link to an explanation about why in D has taken to distinguish between classes and structures?
>>>>
>>>> (Sorry for my bad English)
>>>>
>>>
>>> The main justification is eliminating the slicing problem.
>>>
>>> http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html
>>>
>>> D solves it by making it impossible to have a class instance as a value type.
>>>
>>
>>
>> Why not disallow the casting for structs, leaving the possibility of up casting for the links and pointers to the structure?
>
> What is the best place for such questions?

Here is fine.  digitalmars.d might be ok too.

This seems like a question that deserves a good answer on the "rationale" page: http://www.digitalmars.com/d/2.0/rationale.html

I was just quoting to you the justification I've heard given before. But I'm not necessarily convinced that it's a net win for the language.  I find that it makes things simpler in some ways and more complicated in other ways.  But the "safety first" argument says that if it gets one potential bug (aka slicing), then its a net win.  I'm not so sure.

--bb

Bill Baxter пишет:
> 2008/12/15 Weed <resume755@mail.ru>:
>> Bill Baxter пишет:
>>> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755@mail.ru> wrote:
>>>> Who can provide a link to an explanation about why in D has taken to
>>>> distinguish between classes and structures?
>>>>
>>>> (Sorry for my bad English)
>>>>
>>> The main justification is eliminating the slicing problem.
>>>
>>> http://cplusplusgems.blogspot.com/2005/10/what-is-slicing-problem-class-base.html
>>>
>>> D solves it by making it impossible to have a class instance as a value
>>> type.
>>>
>>
>> Why not disallow the casting for structs, leaving the possibility of up
>> casting for the links and pointers to the structure?
> 
> I have to confess I don't really understand this question.  Can you
> rephrase or give an example?

Sorry, English is not my native language

In C++, we had the problem - "slicing" objects.
In D this problem is solved inability to inherit from structs.
Without inheritance of structs many things are not possible, compared with C++.
Why, instead of the complete inability to inherit, just do not make impossible to up casting struct type by value.

like this:

struct s1 {}
struct s2 : s1 {}

s1 base;
s2 derr;

s1* base_ptr = &derr; // ok
s1 val = derr; // error

Weed пишет:
> Why, instead of the complete inability to inherit, just do not make impossible to up casting struct type by value.

It's a question:
Why, instead of the complete inability to inherit, just do not make impossible to up casting struct type by value?

Weed Wrote:

> Without inheritance of structs many things are not possible, compared with C++.

for example?

Weed wrote:
> In C++, we had the problem - "slicing" objects.
> In D this problem is solved inability to inherit from structs.
> Without inheritance of structs many things are not possible, compared with C++.
> Why, instead of the complete inability to inherit, just do not make impossible to up casting struct type by value.
> 
> like this:
> 
> struct s1 {}
> struct s2 : s1 {}
> 
> s1 base;
> s2 derr;
> 
> s1* base_ptr = &derr; // ok
> s1 val = derr; // error

This is why:
  s1 val2 = *base_ptr; // error

(And disallowing '*ptr' on struct pointers is not likely to find much support)