What's wrong with just a runtime-checked const?

[Bruno Medeiros]

Reiner Pope wrote:
>>> In my view, we either need runtime-checked const or nothing. Consider 
>>> this, and tell me how we can avoid excess dup calls if we use 
>>> C++-style const-checking:
>>>
>>>  class foo {
>>>    private char[] _name;
>>>    public const char[] getName()
>>>    {
>>>      return _name;
>>>    }
>>>    ...
>>>  }
>>>  ...
>>>  Import std.string;
>>>  foo f;
>>>  /*const*/ char[] name = f.getName();
>>>  char[] name_m = tolower(name);
>>>  // Can we modify name_m? Better dup it, just to make sure
>>>  name_m = name_m.dup();
>>
>> if (name_m is name)
>>
> 
> Yes, but try getting that to work with a statically-checked const. It 
> simply won't work unless you have a very smart checker. Of course, you 
> could leave const out altogether, but then we would be making no 
> progress. Let me demonstrate the static const-checking problem:
> 
> char[] tolower(const char[] input) // the input must be const, because 
> we agree with CoW, so we won't change it
> {
>   // do some stuff
>   if ( a write is necessary )
>   { // copy it into another variable, since we can't change input (it's 
> const)
>   }
>   return something; // This something could possibly be input, so it 
> also needs to be declared const. So we go back and make the return value 
> of the function also a const.
> }
> 
> // Now, since the return value is const, we *must* dup it.
> 
> Can you suggest another solution other than avoiding const-checking 
> entirely? By the way, a dedicated string class is actually an 
> implementation of runtime const checking.
>> but, it's certainly not a pretty solution... and there's a related 
>> question - how can tolower know whether to .dup or not.. there are 
>> certainly cases where it isn't necessary.
> This is just another issue that static checking can't solve but runtime 
> checking can (providing we have a modification of the libraries to 
> support a CoW and a in-place version).

I'm not following you. That code example shows how a static const 
checking should work, so what is the problem with that?
And in:
   char[] name_m = tolower(name);
   // Can we modify name_m? Better dup it, just to make sure
You ask if we can modify name_m, the answer is yes if the return type of 
tolower is non-const, no if it const, so what's the issue here?

-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D