logical const without casts!

[Michel Fortin]

On 2011-09-30 18:07:54 +0000, "Steven Schveighoffer" 
<schveiguy at yahoo.com> said:

> On Fri, 30 Sep 2011 14:00:36 -0400, Michel Fortin  
> <michel.fortin at michelf.com> wrote:
> 
>> Interesting proposal. You realize if we had a true 'mutable' storage  
>> class for class members it could obey the same rule as you propose in  
>> that bug report: it should be illegal to cast a mutable-containing  
>> object or struct to immutable implicitly. An explicit cast should be  
>> required.
> 
> Yes, but the benefit of the proposal is -- it already works in the 
> current  compiler :)  You have a monstrous hill to climb to convince 
> Walter to  *add* mutable storage class, but we don't even have to ask 
> for this one,  it works today.

It works today but is unsafe due to the implicit cast. But yeah, lets 
use delegates for now.

After the implicit cast is fixed it might be the right time to point 
out to Walter that this exception for delegate is no worse than having 
a direct mutable keyword. Especially if everyone is using this delegate 
detour to achieve what would be simpler and more efficient with 
'mutable'.


>> Also, if we had shared delegates -- delegates which are guarantied to  
>> only manipulate shared data -- we could allow the class that contains a 
>>  shared delegate to be implicitly converted to immutable.
> 
> I'd like to avoid the complication of having any attributes that apply 
> to  the context pointer.  I like how delegates just fit in anywhere, no 
> matter  what the context pointer type is (or its constancy).

But that's no longer true. Delegates no longer fit anywhere with the 
fix you just proposed. Only 'shared' delegates would fit anywhere. I 
really means *anywhere* since implicitly casting a shared delegate to a 
non-shared delegates is a non-issue: it does not change the type of the 
variables the context is pointing to.

So basically you'd need to require a shared delegate only where a 
normal delegate cannot do the job (like in a class you want to make 
immutable, or wanting to pass the delegate across threads).

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