• Brother Bill (11/11) Feb 18 ```Object foo``` is a class instance, perhaps null, perhaps a
• Nick Treleaven (6/18) Feb 18 Use a cast:
• Brother Bill (10/29) Feb 18 That's it!
• Steven Schveighoffer (10/22) Feb 22 FYI, a very nice idiom for D:

Brother Bill <brotherbill mail.com> writes:

```Object foo``` is a class instance, perhaps null, perhaps a 
Clock or AlarmClock or Vegetable.

What is the expression to ask the boolean question:
   Is foo of type Clock or a type derived from Clock, such as 
AlarmClock?


I tried the obvious ```foo is Clock```, but this did not compile.
```
Error: type `Object` is not an expression
        bool isClock = (o is Clock)
                             ^
```

Nick Treleaven <nick geany.org> writes:

On Wednesday, 18 February 2026 at 14:49:56 UTC, Brother Bill 
wrote:
 ```Object foo``` is a class instance, perhaps null, perhaps a 
 Clock or AlarmClock or Vegetable.

 What is the expression to ask the boolean question:
   Is foo of type Clock or a type derived from Clock, such as 
 AlarmClock?

Use a cast:
https://dlang.org/spec/expression.html#cast_class

 I tried the obvious ```foo is Clock```, but this did not 
 compile.
 ```
 Error: type `Object` is not an expression
        bool isClock = (o is Clock)
                             ^
 ```

That use of `is` is a different expression:
https://dlang.org/spec/expression.html#identity_expressions

Brother Bill <brotherbill mail.com> writes:

On Wednesday, 18 February 2026 at 14:55:50 UTC, Nick Treleaven 
wrote:
 On Wednesday, 18 February 2026 at 14:49:56 UTC, Brother Bill 
 wrote:
 ```Object foo``` is a class instance, perhaps null, perhaps a 
 Clock or AlarmClock or Vegetable.

 What is the expression to ask the boolean question:
   Is foo of type Clock or a type derived from Clock, such as 
 AlarmClock?

 Use a cast:
 https://dlang.org/spec/expression.html#cast_class

 I tried the obvious ```foo is Clock```, but this did not 
 compile.
 ```
 Error: type `Object` is not an expression
        bool isClock = (o is Clock)
                             ^
 ```

 That use of `is` is a different expression:
 https://dlang.org/spec/expression.html#identity_expressions

That's it!
```
const(Clock) clock = cast(const Clock)o;
```

If clock is null, then o is either ```null``` or not derived from 
Clock.
Otherwise o, such as AlarmClock will have access to only the 
Clock features of o.

Steven Schveighoffer <schveiguy gmail.com> writes:

On Wednesday, 18 February 2026 at 15:05:16 UTC, Brother Bill 
wrote:
 On Wednesday, 18 February 2026 at 14:55:50 UTC, Nick Treleaven 
 wrote:
 That use of `is` is a different expression:
 https://dlang.org/spec/expression.html#identity_expressions

 That's it!
 ```
 const(Clock) clock = cast(const Clock)o;
 ```

 If clock is null, then o is either ```null``` or not derived 
 from Clock.
 Otherwise o, such as AlarmClock will have access to only the 
 Clock features of o.

FYI, a very nice idiom for D:

```d
if(auto clock = cast(const Clock)o) {
    // use clock in here as a `Clock` type
}
```

This leaves no chance to use clock as a `null` reference.

-Steve