On Tue, Aug 01, 2017 at 07:31:41PM -0400, Steven Schveighoffer via
Digitalmars-d-learn wrote:
> On 8/1/17 7:15 PM, H. S. Teoh via Digitalmars-d-learn wrote:
> > On Tue, Aug 01, 2017 at 07:09:45PM -0400, Steven Schveighoffer via
> > Digitalmars-d-learn wrote:
> > > If this were a true implementation without the opaqueness, it
> > > would not work properly.
> > [...]
> >
> > Actually, a proper implementation would still work, provided you
> > declare your pointer types carefully. Sketch of idea:
> >
> > auto byKeyValue(AA)(AA aa) {
> > struct Result {
> > const(Slot)* current; // N.B.: proper type
> > bool empty() { ... }
> > auto front() { return Pair(*current); }
> > void popFront() {
> > current = current.next;
> > ...
> > }
> > }
> > return Result(aa);
> > }
> >
> > Basically, the type of `current` must be const(Slot)* rather than
> > const(Slot*), which would be the default inferred type. But since
> > it's legal to assign a const pointer to a pointer to const (you
> > can't modify the original pointer, nor what it points to, but it's
> > valid to copy the pointer to a mutable pointer variable, as long as
> > what is pointed to is still const), this actually will work without
> > breaking / bypassing the type system.
>
> No, you can't const the Slot, because if the value type is a pointer,
> you can't then cast away the const-ness of it legally.
>
> There are ways to get it right, it involves templating for mutability.
[...]
Counter-proof:
struct Slot {
Slot* next;
const(string) key;
const(int) value;
}
struct AA {
Slot*[] slots;
}
unittest {
const(AA) aa;
static assert(is(typeof(aa.slots[0]) == const(Slot*)));
const(Slot)* p = aa.slots[0]; // N.B.: legal
p = p.next; // N.B.: legal
}
Note especially the type checked for in the static assert: you cannot
modify any of the Slot pointers in the AA, but you *can* assign them to
mutable (but tail-const) pointers. Therefore you totally can iterate a
const AA without any casts or any breaking of the type system. And
there's no need to template for mutability either.
T
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