Head Const

[Walter Bright via Digitalmars-d]

On 2/18/2016 9:46 PM, Jonathan M Davis wrote:
> On Thursday, 18 February 2016 at 22:40:32 UTC, Walter Bright wrote:
>> On 2/18/2016 4:16 AM, Jonathan M Davis wrote:
>>> headconst may solve the extern(C++) problem, but it really doesn't solve the
>>> problems we have with const.
>>
>> 'mutable' doesn't really solve a problem, it just means that C++ 'const' is a
>> documentation aid, not a guarantee.
>
> It's still a guarantee for those members that aren't mutable. It's allowing
> casting away const and mutating that totally blows the guarantees out of the
> water.

That's why such casts are not allowed in D @safe code.

C++ const does not come with mechanically checkable guarantees, and D's does. 
This makes all the difference.

Allow @mutable, and no more mechanical checking in D. Recall that D supports 
opaque types, meaning types are not fully known to the compiler.