Frustrations with const
Steven Schveighoffer
schveiguy at yahoo.com
Thu Mar 8 11:50:50 PST 2012
On Thu, 08 Mar 2012 14:49:12 -0500, H. S. Teoh <hsteoh at quickfur.ath.cx>
wrote:
> On Thu, Mar 08, 2012 at 08:22:05PM +0100, Timon Gehr wrote:
>> On 03/08/2012 08:09 PM, H. S. Teoh wrote:
> [...]
>> >The problem is, how to write this function so that it can be called
>> from
>> >*both* a const public method and a non-const public method? Since the
>> >method itself doesn't actually modify anything, it *should* in theory
>> be
>> >possible to mark it as const:
>> >
>> > const Slot *findSlot(Key key) { ... }
>> >
>> >However, because the const applies to 'this', the compiler insists that
>> >referencing anything via 'this', including reading a pointer to a slot,
>> >must also be const, so it refuses to let the return type be Slot*; it
>> >has to be const(Slot)*.
>> >
>> >But this is silly, because now the caller isn't allowed to modify the
>> >Slot either, so now I need to bloat the code with two identical copies
>> >of findSlot, one with const, and one without (since if it wasn't marked
>> >const, then a const method couldn't call it).
> [...]
>> inout(Slot)* findSlot(Key key) inout { ... }
>
> Ahhh. Thanks!
>
> But that still doesn't solve the problem:
>
> inout(Slot)* findSlot(Key key) inout {
> auto slot = slots[hash(key)];
What is type slot, and how is it constructed? This snippit isn't enough
to provide help.
> while (slot) {
> if (slot.hash == hash(key) && slot.key == key)
> return slot;
>
> // Error: cannot modify inout(Slot)*
An exact message is preferrable.
> ---> slot = slot.next;
> }
> return null;
> }
-Steve
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