distinguish between classes and structures

[Weed]

Bill Baxter пишет:
> 2008/12/15 Weed <resume755 at mail.ru>:
>> Bill Baxter пишет:
>>> On Mon, Dec 15, 2008 at 3:35 PM, Weed <resume755 at 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