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