setting a member variable to null

If I assign a value to a new member of the object (i.e. one that didn't exist before), then later assign null to the member....is that the same as if I didn't create it in the first place?  If I don't know whether the object has been created or not, what is the correct way to check for the existance of the object?

For instance, assume the object "thing" is an existing object, but its member "val" has never existed.

thing.val = "hello"; // create member "val" by assigning a value to it

....

// now I am done with it, clear it
thing.val = null;

....

// now I want to check if the object exists
// (i don't care whether or not it USED to exist, I want "never existed" to be
// treated the same as "created and then later set to null")

if (thing.val)
      doSomething (thing.val);
      
or should i do:

if (thing.val != null)
      doSomething (thing.val);
            
or should i do:

if (thing.val && thing.val != null)
      doSomething (thing.val);
            

Which one am I supposed to use?  I want it to work even if the member val has never been created.  And I want it to be the "correct" way.

I am a little confused as to what the "if (thing.val)" means...does it mean the variable is non-null, or that it doesn't exist, or are they effectively the same thing?

I am also confused as to when I would use "delete".  Is setting something to null the same as deleting it?  (note that in many cases, the member might be another object or an array that could use significant memory)Start Free Trial