Null means never existed. Void means it existed but has been made such that it is not in effect. Sort of like when you void a check.

What is the difference between void and null in data structure?

Void refers to the type. Basically the type of data that it points to is unknown. Null refers to the value. It’s essentially a pointer to nothing, and is invalid to use.

Why do they say null and void?

Canceled, invalid, as in The lease is now null and void. This phrase is actually redundant, since null means “void,” that is, “ineffective.” It was first recorded in 1669.