Non-null objects, the Null Object pattern, and T.init

[Ola Fosheim Grøstad" <ola.fosheim.grostad+dlang at gmail.com>]

On Friday, 17 January 2014 at 01:42:38 UTC, Andrei Alexandrescu 
wrote:
> One common idiom to replace null pointer exceptions with milder 
> reproducible errors is the null object pattern, i.e. there is

Usually null failures are hard to track when some function 
returned a null value as an indication of error when the 
programmer expected an exception. In that case early failure is 
better.

I guess it cold make sense with "weak" pointers if they are wiped 
by the GC.

Like if you delete a User object, then all "weak" pointers are 
set to a UnknownUser default object. So that you can call 
user.isLoggedIn() and get false, but you should be able to 
default to a different Class to get a different set of virtual 
calls.

I could also see the use-case for lazy allocation. Basically 
having memory-cheap "smart" pointers that are checked when you 
trap zero-page access and automatically alloc/init an empty 
object before recovering. But the overhead is large.

Not sure if init() is the right way to do it though, because you 
might need something more flexible. E.g. maybe you want access to 
the node that contained the offending null-pointer.