• Mark Isaacson (25/25) Jun 27 2014 If I have a variable whose static type is an interface and I call
• Kapps (13/20) Jun 28 2014 Interfaces are not necessarily Objects (particularly with the
• Steven Schveighoffer (9/27) Jun 29 2014 As I've said many times, instances of D interfaces are necessarily

"Mark Isaacson" <turck11 hotmail.com> writes:

If I have a variable whose static type is an interface and I call
typeid on it, I get the interface back, not the dynamic type.
This seems like confusing behavior. Is this the intended result?

I recognize that one needs some amount of state to perform the
dynamic type lookup, and so it is on that thought that a reason
for this might be based.

My workaround to this issue is shown in the code below, namely,
casting to Object before running typeid produces the expected
result. You can repro the issue by removing that cast.

import std.stdio;
import std.conv;

interface Base {

}

class Derived : Base {

}

void main() {
    Base b = new Derived();
    //1) As is, this prints Derived
    //2) Without the cast, this prints Base -- this is what is
unexpected
    //3) If Base is changed to a class instead of an interface,
this prints Derived regardless of whether or not the cast is in
place
    writeln(text(typeid(cast(Object) b)));
}

"Kapps" <opantm2+spam gmail.com> writes:

On Friday, 27 June 2014 at 21:23:52 UTC, Mark Isaacson wrote:
 If I have a variable whose static type is an interface and I 
 call
 typeid on it, I get the interface back, not the dynamic type.
 This seems like confusing behavior. Is this the intended result?

 I recognize that one needs some amount of state to perform the
 dynamic type lookup, and so it is on that thought that a reason
 for this might be based.

Interfaces are not necessarily Objects (particularly with the 
case of IUnkown or extern(C++)), and are handled somewhat 
differently from objects.

When you cast to Object, you're actually subtracting a few bytes 
from the pointer to get back to the Object, so technically the 
variable refers not to Object but to the interface. It is a bit 
odd (along with some of the other side-effects), but it does make 
some sense since you're referring to the interface and not to an 
Object.

That being said, I'm not 100% sure whether this is the intended 
behaviour when you actually do point to a class derived from 
Object.

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Sun, 29 Jun 2014 01:07:11 -0400, Kapps <opantm2+spam gmail.com> wrote:

 On Friday, 27 June 2014 at 21:23:52 UTC, Mark Isaacson wrote:
 If I have a variable whose static type is an interface and I call
 typeid on it, I get the interface back, not the dynamic type.
 This seems like confusing behavior. Is this the intended result?

 I recognize that one needs some amount of state to perform the
 dynamic type lookup, and so it is on that thought that a reason
 for this might be based.

 Interfaces are not necessarily Objects (particularly with the case of  
 IUnkown or extern(C++)), and are handled somewhat differently from  
 objects.

As I've said many times, instances of D interfaces are necessarily  
Objects. Any D class that implements a D interface is a D object, and  
should implicitly cast to Object.

 When you cast to Object, you're actually subtracting a few bytes from  
 the pointer to get back to the Object, so technically the variable  
 refers not to Object but to the interface. It is a bit odd (along with  
 some of the other side-effects), but it does make some sense since  
 you're referring to the interface and not to an Object.

Correct. It would be a trivial extra-step of subtraction to get the  
TypeInfo of the class itself.

 That being said, I'm not 100% sure whether this is the intended  
 behaviour when you actually do point to a class derived from Object.

It is the intended behavior. And I think your reasoning of the IUnknown  
problem is the intended reasoning. And I think it's wrong :)

-Steve