Walter Bright <newshou...@digitalmars.com> wrote:
immutable - Data will never, ever change, through any reference.
const - Data will never change through this reference.
newlevel - Data will never logically change through this reference.
The problem with newlevel is (to repeat myself) it is unverifiable.
As long as it is transitive, I believe it is verifiable:
struct S {
int n;
}
struct foo {
int n;
foo* p;
newlevel S s;
newlevel int m;
void bar( ) newlevel {
m++; // works, m is newlevel
n++; // compile-time error, n is const for newlevel this
s.n = 3; // Works, s.n is newlevel
p.n = 4; // compile-time error, p is newlevel, p.n is const for
// newlevel this
p.m++; // Works, p.m is newlevel
}
void baz( ) const {
m++; // compile-time error, m is const for const this
}
}
void qux( newlevel foo f ) {}
void quux( ref foo f ) { f.n++; }
void main() {
const foo f;
f.bar( ); // compile-time error, f is const, cannot call newlevel
// functions on it
qux( f ); // compile-time error, const(foo) cannot be implicitly
// cast to newlevel(foo)
newlevel foo g;
quux( g ); // compile-time error, newlevel(foo) cannot be implicitly
// cast to foo
}
This leaves const as a strong, transitive guarantee, and newlevel as a
transitive guarantee that only newlevel members may be changed.
I am not convinced that such an extension of the type system should be
made, but I believe you are wrong in that it is not verifiable. but
please, do show me my mistake.
--
Simen
