On Monday, 12 November 2012 at 03:32:52 UTC, Vijay Nayar wrote:
I was working on a little project when I encountered something
odd. Essentially a base class contains some data and a derived
class needs a slice of that data. Certain properties of this
slice must be maintained, so the derived class has an invariant.
What happens is that if the base class, even in the
constructor, calls a function that is overridden in the derived
class, the invariant gets invoked. This can make
initialization quite difficult.
Below is a stripped down example of this.
void main() {
auto b = new B([1, 2, 3, 4, 5, 6]);
}
class A {
uint[] data;
this(uint[] data) {
this.data = data;
init(); // This ends up calling B.init() and fails the
invariant!
}
void init() {
// Do some checking.
}
}
class B : A {
uint[] dataSlice;
invariant() {
assert(dataSlice !is null);
}
this(uint[] data) {
super(data);
dataSlice = data[3..$];
}
override void init() {
// Do more checking;
super.init();
}
}
If the base class constructor needs to occur before the derived
class constructor, is there a good way to solve this without
getting rid of the invariant?
- Vijay
Make init private and don't override it.
