How to iterate over const(RedBlackTree)?

[monarch_dodra via Digitalmars-d]

On Friday, 17 October 2014 at 01:09:00 UTC, John McFarlane wrote:
> On Friday, 3 January 2014 at 07:22:32 UTC, monarch_dodra wrote:
>> On Thursday, 2 January 2014 at 14:59:55 UTC, Adam D. Ruppe 
>> wrote:
>>> On Thursday, 2 January 2014 at 13:30:06 UTC, monarch_dodra 
>>> wrote:
>>>> Currently, this is not possible. Or if it was, it would have 
>>>> a
>>>> *very* high code cost inside RBT.
>>>
>>> It is possible in theory; RBT returns a separate Range type 
>>> with opSlice that is implemented in terms of pointers to 
>>> node. (This is hidden by an alias RBNode!Elem* Node, which 
>>> gets in the way, but if those were inout(RBNode)* or const()* 
>>> it'd work).
>>>
>>> We can have mutable pointers to const data, which would work 
>>> with the range. So opSlice returns a mutable range that 
>>> points back to const data.
>>>
>>>
>>> But in this case, none of the functions in rbtree use const 
>>> nor inout, and there's some caching (e.g. _left and _right) 
>>> that I'm not sure can work with it at all anyway. In a const 
>>> node, the left and right properties won't work..
>>
>> Right. Doable, but not trivially so :/
>>
>> Array might be able to pull it off more easily.
>>
>> That said, it would only solve the "const container => Range"
>> issue, but the "const range" problem itself would remain :(
>
> I'm trying to get to grips with D, coming from a C++ background 
> and const correctness is tough thing to get comfortable with. 
> I've got a member variable that's RedBlackTree and I'd like to 
> examine its contents inside an invariants() block. Is this 
> possible at all right now? Obviously, this would be trivial to 
> achieve using std::set::const_iterator so is it the language or 
> the library that poses the difficulty for me? Thanks.

Currently, D containers don't offer "ConstRange opSlice() const" 
(which would be the equivalent of const_iterator). This could be 
a good solution the the issue.

But to answer your question: Both the language and library that 
are making your life difficult. For starters, the D language does 
not use const the way C++ does, so this usually confuses the 
zealous newcomers that want to be "const correct". The library is 
also getting in your way in that it does not provide support for 
"const(container/range)" nor "container of const".