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.
