Nullable or Optional? Or something else?

[Rainer Deyke]

Andrei Alexandrescu wrote:
> Rainer Deyke wrote:
>> You need some syntactic way to distinguish the contained value from the
>> container.  Using "alias this" seems messy here.  Optional!Optional!T is
>> both valid and likely to occur.
> 
> Interesting. I wonder whether it's better to fold Optional!(Optional!T)
> into Optional!T.

That would be semantically incorrect.  Optional!T must be able to hold
all possible values of T, plus a distinct null value.

Consider:

  Optional!int upper_limit;

  Optional!(Optional!int) config_limit = config_file.get("upper_limit");
  if (isNull(config_limit)) {
    upper_limit = default_upper_limit;
  } else {
    upper_limit = config_limit; // Could be null.
  }

Then consider that this could be in a template function, with
Optional!int supplied as a template argument.


-- 
Rainer Deyke - rainerd at eldwood.com