Logical const

[Steven Schveighoffer]

On Mon, 29 Nov 2010 18:04:31 -0500, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> Steven Schveighoffer wrote:
>> On Mon, 29 Nov 2010 15:58:10 -0500, Walter Bright  
>> <newshound2 at digitalmars.com> wrote:
>>
>>> Steven Schveighoffer wrote:
>>>> Having a logical const feature in D would not be a convention, it  
>>>> would be enforced, as much as const is enforced.  I don't understand  
>>>> why issues with C++ const or C++'s mutable feature makes any  
>>>> correlations on how a D logical const system would fare.  C++ const  
>>>> is not D const, not even close.
>>>
>>>
>>> Because people coming from C++ ask "why not do it like C++'s?"
>>  I don't get it.  A way to make a field mutable in a transitively-const  
>> system is syntactically similar to C++, but it's not the same.  Having  
>> a logical-const feature in D does not devolve D's const into C++'s  
>> const.  If anything it's just a political problem.
>
> Having mutable members destroys any guarantees that const provides.  
> That's not political.

What guarantees?  Const provides no guarantees.

class C
{
    static C theCommonOne;
    int x;
    void foo() const {
       theCommonOne.x = 5; // theCommonOne could be this
    }
}

Only immutable provides guarantees.  As I said earlier, the place where  
logical const would cause issues is immutable.  Immutable data is  
currently implicitly shareable, and the compiler may take steps to  
optimize strongly-pure function calls, which require immutable  
parameters.  Any logical const system would have to either work with those  
requirements or force them to be reevaluated.

> And, I repeat, having a mutable type qualifier DOES NOT make logical  
> const a language feature. This is why discussion and understanding of  
> C++'s const system is so important - people impute characteristics into  
> it that it simply does not have.

I've proven in the past that logical const is equivalent to full const.   
That is, I can emulate logical const without any casts in the current  
const regime.  Making it more efficient and simpler is all we would be  
doing.

-Steve