D2.0: an example of use-case for casting invariant away

[Don Clugston]

Walter Bright wrote:
> Daniel Keep wrote:
>> Walter's said on a few occasions that the problem with C++'s const is
>> that casting away const is a *well-defined* operation.  That means that
>> const cannot be used to do any kind of optimisation, since it doesn't
>> really mean anything.
>>
>> The new page on const specifically states that casting away const or
>> invariance is *not* a well-defined operation, and you'd better know what
>> you're doing when you do it.
> 
> More specifically, in C++, you can cast away const-ness and then modify 
> the underlying data, and this is defined to be legal, defined behavior.
> 
> With D, you can cast away const-ness, that is legal. But if you 
> subsequently modify the underlying data, that is undefined behavior.

It sounds that in D, it will be too easy to cast away constness accidentally.
With C++, at least you can grep for const_cast and detect potentially dangerous 
code, and you get a strong visual clue.
Suppose I've written a D function like this:

void f(int *b, uint c)
{
   // maybe I'm avoiding a compiler warning or something.
   uint *d = cast(uint *)b;
   d += c;
}

Months later, I'm refactoring the code, and I convert the int * parameter to an 
invariant, without recognising that it's changing the value of b. Oops.

C++'s const would catch this mistake, but if I understand correctly, D will 
compile it without error. Suddenly the function has moved into the realm of 
undefined behaviour.

I hope I'm wrong. Or did I miss something?