On Monday, 20 June 2016 at 00:15:33 UTC, David Nadlinger wrote:
On Monday, 20 June 2016 at 00:01:58 UTC, Joerg Joergonson wrote:
class a { }
class b : a { }
class A(T : a)
{
T x;
}
void main(string[] argv)
{
auto y = new A!a();
auto z = new A!b();
y.x = new a();
z.x = new b();
auto p1 = cast(A!a)y;
auto p2 = cast(A!b)z;
auto p3 = cast(A!a)z; // Fail! Why? z.x is of type b which
can be down cast to type a without a problem ->
As has been pointed out in dm.D.learn already (which is where
this discussion belongs), there isn't any inheritance
relationship between A!b and A!a.
In your example, it's easy to see that there can't be if you
ask yourself whether the purported relation should be co- or
contravariant in nature. To put it in simple terms, if your
cast succeeded, you could do this:
---
p3.x = new a;
// Now typeof(z.x) == b, but it points to an instance of a!
---
— David
So?
auto f = 1f;
auto x = cast(int)f;
f is float, x is an int, that's what casts do when you cast
beyond their type. I mean, after all, p3.x is of type A!a and
points to the same location as something that maps to A!b. It's
no doubt going to be a problem if you stick something less in
there than you were suppose do.
If the point is: Too keep programmers from making mistakes. Then
that's fine. But that doesn't mean in all cases is is bad.
Sure I can down cast to a ButtonItem then store ButtonItem's in
something that's actually a SliderItem, which is effectively
upcasting behind the scenes... But that's my fault. It can be
done outside inheritance too. Take a pointer to an float and
write an int value to it... Same problem.
If this is a restriction, then fine. Then the question is, how to
get around it. I don't do anything that causes problem. I only
iterate over the base type and never assign a base type to
something that might contain a derived type.
Can I just force the cast in some way if I know good an well it
works for ever? Or do I have to write extra code to get around
axiom: "We did this to save you trouble because we know better
than you what you are doing. Go forth and write more code!"?
