The Status of Const

[Michel Fortin]

On 2010-08-16 09:08:30 -0400, "Steven Schveighoffer" 
<schveiguy at yahoo.com> said:

> On Mon, 16 Aug 2010 08:56:51 -0400, Michel Fortin  
> <michel.fortin at michelf.com> wrote:
> 
>> The idea's not bad, but I think it needs some improvments. For 
>> instance,  in a struct, I might want some members to be part of the 
>> tail and some  other not. I believe that should be allowed somehow.
> 
> What do you mean "not", meaning they are fully const?  Because you 
> can't  not apply const to references inside a const struct, that's 
> logical const,  and while it might be a nice feature, Walter has 
> steadfastly refused to  allow it.

I'm not arguing for logical const. Logical const means that you have an 
island of non-const inside a const structure. That's not what I meant.

When you want your struct to be tail-const, what it means is that you 
want the members inside it to be tail-const. The structure itself is 
not const, just the members, an just the tail of the members.

But do you always want *all* the members to be tail const?

Say your struct is a reference counting smart pointer that works by 
having two pointers: one points to the actual object, the other points 
to the reference counter (boost::shared_ptr's design). This smart 
pointer basically has two tails, one for each internal pointer. But if 
you apply "tail const" to the smart pointer, you probably only want the 
pointer to the actual object to be tail-const. That's because making 
the pointer to the reference counter tail-const would just prevent you 
from updating the counter, which in turn would prevent you from 
assigning something different to the smart pointer.

I think what you want for that is to somehow make SmartPtr!(X) 
implicitly derived from SmartPtr!(const X), you don't want the compiler 
applying blindly tail-const to all the members.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/