Logical const

[Michel Fortin]

On 2010-11-29 11:24:56 -0500, "Steven Schveighoffer" 
<schveiguy at yahoo.com> said:

> On Mon, 29 Nov 2010 10:56:14 -0500, Andrei Alexandrescu  
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> In fact it's possible to provide logical const with guarantees. You 
>> need  a construct that keeps together a bool, a pure 
>> function/delegate/method,  and a value, and works like this:
>> 
>> class X
>> {
>>      int x;
>>      lconst int y = { return x + 42; };
>>      void m1() { x = 2; invalidate(y); }
>>      void m2() const { writeln(y); }
>> }
>> 
>> You can use the intrinsic invalidate() against a non-const lconst 
>> field,  but not against const lconst. Reading that field checks whether 
>> the  field has been invalidated. If so, it writes the field with the 
>> result  of the function/delegate/method and turns the validation flag 
>> on.
>> 
>> With this system in place, I think the lconst value is guaranteed to be 
>>  as strong as const.
> 
> This only solves the caching issue (which I agree is a large issue).   
> There are other cases where you want a mutable pointer to other data 
> that  the object does not own.  Such as a widget having a pointer to 
> its  window.  Assuming that window draw routines need to be non-const, 
> a widget  cannot draw itself.  You need to keep the widget to window 
> relationship  outside the class, or cast.  Both of these solutions are 
> awkward at best.

You can pass the drawing context (the window in your case) as an 
argument do the draw function.


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