what if return returned subclasses

H. S. Teoh hsteoh at quickfur.ath.cx
Thu Oct 21 00:01:24 UTC 2021

On Wed, Oct 20, 2021 at 10:42:43PM +0000, Adam D Ruppe via Digitalmars-d wrote:
> ok consider this:
> Object identity(return Object o) { return o; }
> Subclass s = identity(new Subclass());
> That is, it is converted to the base class before passed to the
> function, but since it is marked `return` anyway, the compiler knows
> it is the same instance, so the static type passed in is retained in
> the return value. So it is kinda like type erased but then repainted
> back on.
> Would this break anything? There's a few OOP patterns that might be
> made a lil nicer if this just worked.

What would you do about this case?

	Object choice(return Object a, return Object b, int cond)
		return (cond) ? a : b;

	SubclassA a;
	SubclassB b;
	a = choice(a, b, 1);	// what if you assigned to b instead, or cond == 0?


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