Detecting if a class type (which may or may not have a default constructor) is abstract

[Jarrett Billingsley]

On Thu, Nov 13, 2008 at 8:50 PM, Christopher Wright <dhasenan at gmail.com> wrote:
> Jarrett Billingsley wrote:
>>
>> I just don't think it's possible.  If all classes had default ctors,
>> it'd be easy; is(typeof(new T)) would be false if and only if T were
>> abstract.  But since that's not the case, I can't think of a way to
>> generically see if a given class type is abstract.  Any ideas?
>>
>> It's always a little frustrating when doing type introspection and
>> having to rely on weird side-effects and properties of types, when the
>> compiler is just keeping it in some flag or field somewhere.  Sigh.
>> "is(T == abstract)"?  :P
>
> If you know the constructor arguments in advance, you can do something like:
> static if (is (typeof (new Foo (1, "hello")))){}

Oh, definitely.  But I'm writing a library where the ctor signatures
are provided by the user, and "new T(InitsOf!(Types))" could fail
either because T is abstract or because they just gave an invalid
signature

> Unfortunately, ParameterTupleOf!(T._ctor) doesn't work:
>
> class AFoo {}
> if (is (typeof (AFoo._ctor))) Stdout.formatln ("AFoo._ctor");
> if (is (typeof (ParameterTupleOf!(AFoo._ctor)))) Stdout.formatln
> ("AFoo._ctor params");
> // prints AFoo._ctor
>
> _ctor is a really odd construct -- spotty support, not advertised.

I wish it worked right.  Constructors are always the odd ones out.
They're just functions, and should be introspectable as such.