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

[Reiner Pope]

Bruno Medeiros wrote:
>> 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?

You seem to have ignored my more recent example. The problem is that 
static checking *forces* the return value to be const, which in turn 
forces you to dup it. You don't always need to do that:

 >> 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.
 >>