Base Class Hitting Derived Class Invariant

[Tobias Pankrath]

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.