I'm writing an AA implementation, and ran into this problem with the way const behaves in D. Basically, I have an auxiliary function that searches the internal hash table for a given key, and returns the slot containing the matching entry, if found.

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).

Is there any way to work around this?

Or more importantly, why does the compiler force all internal members of this to be const inside a const method, when what it really should be enforcing is that nothing is being *modified*? What's wrong with extracting non-const members from the object in a const method, if you never actually do anything to it?


T

-- 
Never wrestle a pig. You both get covered in mud, and the pig likes it.

On 03/08/2012 08:09 PM, H. S. Teoh wrote:
> I'm writing an AA implementation, and ran into this problem with the way
> const behaves in D. Basically, I have an auxiliary function that
> searches the internal hash table for a given key, and returns the slot
> containing the matching entry, if found.
>
> 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).
>
> Is there any way to work around this?
>
> Or more importantly, why does the compiler force all internal members of
> this to be const inside a const method, when what it really should be
> enforcing is that nothing is being *modified*? What's wrong with
> extracting non-const members from the object in a const method, if you
> never actually do anything to it?
>
>
> T
>

inout(Slot)* findSlot(Key key) inout { ... }