const debacle

[Steven Schveighoffer]

"Walter Bright" wrote
> Steven Schveighoffer wrote:
>> I want to specify that the function does not change the input buffer, and 
>> returns the same type as passed in (without duping the input).
>
> Look at it another way. You want to declare a filter that does not change 
> the input, but can pass it along to another function that will. I suspect 
> this might be a fundamentally broken concept:
>
>    T[] f(const T[]);
>    void g(T[]);
>
>    ...
>
>    T[] t;
>    g(f(t));
>
> What's happened here? Although f() promised it wouldn't modify t, it got 
> modified anyway, because f() passed t to g() which modified it. Far from 
> being a weakness in D's const regime, I think the fact that such cannot be 
> implemented without casting away const at some point shows that the const 
> system is sound, and trying to implement such a filter is unsound.

I wholeheartedly disagree :)

g(f(t)) is a shortcut for:

auto tmp = f(t);
g(tmp);

and as you can see, using g on the output of f is not f's fault.  Even if 
you wanted a filter function, I would expect the filter to return exactly 
the same type as it was given (as it is meant to be injected in place as an 
argument to another function which would normally take the same input).

Actually, your example demonstrates this point exactly.  If the code looks 
like:

g(t);

and I say "oh, I want to filter that input with f":

g(f(t));

If f now returns const T[], then this will not compile, and the filter is 
useless, but here comes the worst problem.  If someone else is responsible 
for f, they might say "hey, f doesn't modify the input, so I can re-label it 
const.  Oops, it won't let me return a mutable array, I'll just change that" 
without realizing the implications it has on its usage.

I think people are going to have to leave out const for such fucntions and 
templatize them, even if they are correctly const, and therefore, const 
loses some of its value (not to mention, if you templatize f, it can 
generate 2 or 3 identical functions for no good reason).

I have been thinking of a way to specify this type of function, but it's not 
completely cooked, when it is, I'll post my idea.

-Steve