const again

[Janice Caron]

On 12/6/07, Walter Bright <newshound1 at digitalmars.com> wrote:
> There are a couple problems with it, the worst of which is its impact on
> generic code:
>         const(T)[]
> Would you put the & there or not?

Good point. You'd need it for a class, but not for a struct. I hadn't
thought of that.

That said, would that be much of a problem? Arrays of structs are very
different beasts from arrays of class references. The copy semantics
alone are different enough that one might imagine that generic code
would need one or two if(is(T==struct))s in there anyway.


> What would & mean if one wrote:
>         struct S { C c; }
>         const(S)&[] a;
> ?

I had in mind that that would be a syntax error.



> One principle we try to adhere to is that it should make sense to be
> able to wrap any type with a struct, and have it be possible for that
> struct to behave as if it were that member type.

If we went with the & syntax, then that principle would dictate the
requirement of an additional operator overload, which for want of a
better name I shall temporarily call opAmpersand.

(...which begs the question, which of opStar and opAmpersand has more
right to be called opDeref... but let's worry about that later!)



> And finally, this suggests that & means "tail-const".

I prefer to think that it means "reference to", in the same way that *
means "pointer to". However, this is D, not C++, so the symbol would
only be allowed for types which were /already references/, in which
case it could be used to indicate tail constness by placing the symbol
outside the brackets. Note that const(C&) would mean exactly the same
thing as const(C) - so it's not the ampersand that means tail-const,
it's its placement.


> Tail-const has
> that severe problem that there is no such thing as a tail-const member
> function (i.e. a member function that can modify the fields of the
> object, but not anything those fields refer to).

Forgive me - perhaps I haven't thought this through deeply enough. I
don't follow why that's a problem.