http://d.puremagic.com/issues/show_bug.cgi?id=6857
--- Comment #56 from Don <clugd...@yahoo.com.au> 2012-05-06 00:36:04 PDT ---
(In reply to comment #55)
> Mea culpa.
>
> I read Meyer's book again. Chapter 16.1 "Cutting out the middleman" pg. 575
> says:
>
> "A client of MATRIX must satisfy the original (stronger) precondition, and may
> only expect the original (weaker) postcondition; even if its request gets
> served dynamically by NEW_MATRIX it has no way of benefiting from the broader
> tolerance of inputs and tighter precision of results. To get this improved
> specification it must declare the matric to be of type NEW_MATRIX, thereby
> losing access to other implementations represented by descendants of MATRIX
> that are not also descendants of NEW_MATRIX."
>
> (MATRIX is the base class, NEW_MATRIX is the derived class.)
>
> So I'm reopening it as a normal bug.
>
> Unfortunately, I do not currently see a reasonable way of implementing this.
> Fortunately, as is it does not inhibit correct programs, it only accepts some
> invalid ones.
I think implementation is simpler than what happens at present. You need a
static function thunk for each virtual function with a precondition.
For example:
class A {
int foo(int n) in { assert(n >0); } body { return n; }
}
class B {
int foo(int n) { return n+1; } // no change to precondition
}
class C {
int foo(int n) in { assert(n > -10 && n<-6); } body { return n+2; }
becomes (I've put 'virtual' in to make things clearer):
class A {
static void foo_in(A x) { assert(n>0); }
virtual int foo(int n) { foo_in(this, n); return foo_body(n); }
virtual int foo_body(int n) { return n; }
}
class B {
virtual int foo_body(int n) { return n+1; }
}
class C {
static void foo_in(C x, int n) { assert(n > -10 && n<-6); }
virtual int foo(int n ) {
if (C.foo_in(this, n)) return foo_body(n); // passed C's precondition
return A.foo(n); // failed, try A's precondition
}
virtual int foo_body(int n) { return n+2; }
}
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