Re: Open question: what code pattern you use usually for null safety problem

[Steven Schveighoffer via Digitalmars-d-learn]

On 1/14/21 7:27 PM, ddcovery wrote:
On Thursday, 14 January 2021 at 20:23:08 UTC, Steven Schveighoffer wrote:

You could kinda automate it like:

struct NullCheck(T)
{
   private T* _val;
   auto opDispatch(string mem)() if (__traits(hasMember, T, mem)) {
       alias Ret = typeof(() { return __traits(getMember, *_val, mem); 
}());
       if(_val is null) return NullCheck!(Ret)(null);
       else return NullCheck!(Ret)(__trats(getMember, *_val, mem));
   }

   bool opCast(V: bool)() { return _val !is null; }
}

auto nullCheck(T)(T *val) { return AutoNullCheck!T(val);}

// usage
if(nullCheck(person).father.father && person.father.father.name == 
"Peter")

Probably doesn't work for many circumstances, and I'm sure I messed 
something up.

-Steve

I'm seeing "opDispatch" everywhere last days :-). It's really powerful!!!

If we define an special T _(){ return _val; } method, then you can write

   if( nullCheck(person).father.father.name._ == "Peter")

And renaming

   if( ns(person).father.father.name._ == "Peter" )

This doesn't work, if person, person.father, or person.father.father is 
null, because now you are dereferencing null again.

But something like this might work:

NullCheck(T)
{
   ... // opdispatch and stuff
   bool opEquals(auto ref T other) {
      return _val is null ? false : *_val == other;
   }
}

Something similar to BlackHole or WhiteHole. Essentially there's a 
default action for null for all types/fields/methods, and everything 
else is passed through.

Swift has stuff like this built-in. But D might look better because you 
wouldn't need a chain of question marks.

-Steve