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 14:13:40 UTC, Adam D. Ruppe wrote:
> bool isLoggedIn(User user) {
>     return user is null ? false : user.isLoggedInImpl();
> }

In most cases you can avoid null-related issues if you plan for 
it, but if you adapt existing software you might not afford to do 
the changes. Even simple adaption of a framework can cost many 
weeks of hard work and then you need to keep it in sync with 
future versions of the framework.

I also think there might be cases where you run the null-test a 
lot of times with the same result and would be better without it. 
Like in deep nested data structures where the pointer almost 
never is null, and then a trap with recovery might be cheaper, 
amortized.

Sometimes you can write more efficient datastructures if you ban 
deletion:

Like:
ptr = someobject;
weakcontainer.add(ptr);
...
ptr.setdeleted();
ptr = null;
...
weakptr = weakcontainer.get(id);
if(weakptr.isValid())...;

In this case weakptr would point to the original object before 
GC, and to the init object after GC. Saving space, but not 
requiring changes to the weakcontainer implementation.