Casting away const

[Steven Schveighoffer]

On Mon, 09 Aug 2010 10:53:48 -0400, BCS <none at anon.com> wrote:

> Hello Steven,
>
>> On Mon, 09 Aug 2010 10:37:14 -0400, BCS <none at anon.com> wrote:
>>
>>> Hello Steven,
>>>
>>>> On Mon, 09 Aug 2010 10:11:39 -0400, Don <nospam at nospam.com> wrote:
>>>>
>>>>> Steven Schveighoffer wrote:
>>>>>
>>>>>> On Sun, 08 Aug 2010 17:56:25 -0400, simendsjo
>>>>>> <simen.endsjo at pandavre.com> wrote:
>>>>>>> I'm totally new to the const/immutable thing, so this might be a
>>>>>>> naive  question..
>>>>>>> The spec says:
>>>>>>> "modification after casting away const" => "undefined behavior"
>>>>>> I thought it was "you're on your own", not undefined behavior.
>>>>>> The
>>>>>> former implies there is some "right" way to do this if you know
>>>>>> more
>>>>>> about the data than the compiler, the latter implies that there is
>>>>>> no
>>>>>> right way to cast away const.  Am I wrong?
>>>>>> -Steve
>>>>> I think you're wrong. It's OK to cast away const, as long as you
>>>>> don't
>>>>> modify the thing which is supposed to be const.
>>>>> But if you modify it, there's no way the compiler can guarantee
>>>>> that
>>>>> your code will work. Anything could happen.
>>>> But then what is the point of casting away const?  If you are not
>>>> going to  modify it, then there is no reason to cast it away.
>>>>
>>> There are some cases where non-const pointers are used but never
>>> modified (C api's for instance) cast as always is just a way subvert
>>> the  type system where it gets in your way.
>>>
>> C's api can be modified at declaration.  It has no mangling, so you
>> can  type it how it should be (if C had const).  For example:
>>  extern(C) int strlen(const(char)* str);
>>  I find that much more pleasant than having to cast away const.
>
> OTOH that is effectively a hidden cast and has 100% of the same issues  
> (re undefined behavior) as casting away const while being slightly  
> harder to find.

But you just said that casting and reading is not undefined?  Isn't this  
the same thing?

Const is such a strange beast because it plays no role in code generation,  
it's effectively only a tool to help the compiler decide what is  
possible.  It doesn't occupy any space or translate whatsoever to the  
underlying code.

I think there are definite good uses for writing to const or immutable  
data besides ones that can be stored in ROM.  That is, if you are sure a  
const or immutable piece of data is on the heap/stack, it should be  
reasonable to be able to modify it for performance gains.

-Steve