typeid() broken for interfaces?

[foobar]

On Tuesday, 4 December 2012 at 12:30:29 UTC, Paulo Pinto wrote:
> On Tuesday, 4 December 2012 at 12:26:53 UTC, Jacob Carlborg 
> wrote:
>> On 2012-12-04 09:22, Maxim Fomin wrote:
>>
>>> And what happens if nobody implements an interface?
>>>
>>> import std.stdio;
>>>
>>> interface I { }
>>>
>>> class A { }
>>>
>>> void main()
>>> {
>>>    I i;
>>>    // assume this is implicit
>>>    Object o = cast(Object)i;
>>>    writeln(typeid(o));
>>> }
>>
>> You get a segmentation fault since both "i" and "o" are null.
>>
>>> Safe conversion class to interface requires two conditions:
>>> 1a) that class implements interface
>>> 1b) if you try to use interface variable, it must be an 
>>> allocated class
>>> instance
>>>
>>> Safe conversion to Object requires:
>>> 2a) somebody in class hierarchy implements interface
>>> 2b) interface instance is actually allocated class instance
>>
>> You cannot really get an instance of an interface without 
>> having a class implementing it. That is, without inserting any 
>> explicit casts, which works:
>>
>> interface I { }
>>
>> class A { }
>>
>> void main()
>> {
>>    A a = new A;
>>    I i = cast(I) a;
>>    Object o = cast(Object)i;
>>    writeln(typeid(a)); // A
>> }
>>
>>> It is possible to check 1a) but impossible in general case to 
>>> check 2a).
>>> Also the first is design feature while the second is design 
>>> abuse.
>>
>> I don't understand why it wouldn't be safe to allow implicit 
>> casts of interfaces to Object.
>>
>> If I want to call toString, why should I need to insert an 
>> explicit cast to Object just because I have an interface?
>
> If you want to call toSring, it should be part of the interface 
> specification, as simple as that.
>
> Interfaces are like contracts which state what is to be 
> expected of a certain type.
>
> --
> Paulo

Generally speaking you are right. But specifically regarding 
toString, what would you suggest?
Should each and every Interface have a toString method?
Should we have an IObject Interface that all classes are required 
to explicitly inherit?
The purpose of having a root Object class is to define the 
_common interface of all objects_. Why else have such a class in 
the first place?
Requiring an explicit cast to Object makes sense only in a 
no-single-root design such as the one in c++ which D sensibly 
chose not to copy.

We can have a separate debate what should the root Object define, 
but there should not be a requirement to explicitly cast any 
object to it.