how to compare the type of a subclass

[Eric via Digitalmars-d-learn]

On Friday, 21 November 2014 at 22:52:54 UTC, H. S. Teoh via 
Digitalmars-d-learn wrote:
> On Fri, Nov 21, 2014 at 10:30:51PM +0000, Eric via 
> Digitalmars-d-learn wrote:
>> On Friday, 21 November 2014 at 22:25:32 UTC, anonymous wrote:
>> >On Friday, 21 November 2014 at 22:15:37 UTC, Eric wrote:
> [...]
>> >>Is there a way I can check the type of x without doing
>> >>a string comparison?
>> >>
>> >>-Eric
>> >
>> >assert(typeid(x) == typeid(Y));
>> 
>> Thanks.  That works.  I have't quite mastered the whole
>> is/typeof/typeid thing yet.
>
> Binary `is` (i.e., `a is b`) is for comparing two references for
> equality. It returns true if two class reference point to the 
> same
> object; false otherwise. But you probably already know this.
>
> Unary `is` (`is(...)`) is a monstrous maze of obscure syntax, 
> that's
> best learned on a case-by-case basis. :-) But in this 
> particular case,
> it's basically to provide a context for comparing types, since 
> types
> aren't actual objects at runtime. So you can't write `if 
> (typeof(a) ==
> typeof(b))`, for example, because types aren't objects. 
> Instead, you
> have to write `if (is(typeof(a) == typeof(b)))`.
>
> `typeof` is to extract types at compile-time. It returns the 
> *declared*
> type of the variable (which may not be the most derived class 
> if you
> assigned a derived object to a base class reference).
>
> `typeid` is to introspect types at runtime. It returns the most 
> derived
> type of the object, even if the declared type is a base class. 
> The
> returned type is an actual runtime object -- since at 
> compile-time, the
> most derived type may not be known, so it's not representable 
> as an
> actual type at compile-time. Instead, the D runtime returns a 
> TypeInfo
> object that corresponds with the runtime type of the object. So 
> you
> don't need to (and shouldn't) use `is(...)` when comparing 
> typeid's.
>
> In short:
>
> To compare (declared) types at compile time:
>
> 	is(typeof(x) == typeof(y))	// x has the same (declared) type 
> as y
> 	is(typeof(x) : typeof(y))	// x implicitly converts to y
>
> To compare (actual) types at runtime:
>
> 	typeid(x) == typeid(y)	// x has the same (actual) time as y
> 	cast(B)x !is null	// x is a derived class instance of base 
> class B
>
> There are many other cases, of course, but these are the 
> pertinent ones
> in the context of your original question.
>
>
> T

Thanks, this helps a lot.

-Eric