I think this is about 2 problems coming together:

- 1: In C (and relatives), you cannot rule out that any pointer is not-null. Simply due to pointer arithmetic.

- 2: Some values have no default-zero state.

On #2 I found the EG discussions on the Java mailing lists to be fascinating. Think of innocuous types such as "date", where a default value only causes problems. You end up with something like "1970:0:0" or "0:0:0" and it acts like a valid date, but its use is likely unintentional and leads to follow-up issues.

And once you have multi-threading involved, even a simple null-flag becomes a difficult technical challenge to implement under all conditions.