On Friday, 20 February 2015 at 10:41:04 UTC, Jonathan M Davis
wrote:
class Super {}
class Sub : Super {}
class Sub2 : Super {}
Sub[] subArr = createSubArr();
Super[] superArr = subArr;
superArr[2] = new Sub2;
Now, all of a sudden, subArr[2] is a Sub2, when a Sub2 is not a
Sub. So, you
would have broken the type system. The issue is called array
covariance.
const avoids the problem, because you can't assign any of the
elements of
the array.
A simple cast will always permit to destroy the type system
entirely:
class Super {}
class Sub : Super {
string alpha() { return "I am Sub"; }
void SomeMethodNotFoundInSub2() { }
}
class Sub2: Super {
string beta() { return "I am Sub2"; }
}
void foo(in Super[] sup)
{
cast(Super)sup[0] = new Sub();
cast(Super)sup[1] = new Sub2();
}
int main(string[] argv)
{
Sub[] subArr = new Sub[2];
foo(subArr);
writeln(subArr[0]);
writeln(subArr[1]); //look ma', my Sub[] array contains Sub2
elements
writeln(subArr[0].alpha());
writeln(subArr[1].alpha()); //look ma', I'm calling beta of
Sub2, because it has the same vtbl offset as alpha in Sub
subArr[0].SomeMethodNotFoundInSub2(); //ok
subArr[1].SomeMethodNotFoundInSub2(); // now we have an
AccesViolation or maybe not, depending on Sub2 contents.
getchar();
return 0;
}
The problem is in fact the line below, and that kind of
assignment must be checked at runtime instead of limiting compile
time features that can be anyway circumvented, by throwing a
specific exception.
sup[1] = new Sub2();
