The null Pointer

null is a pointer that addresses nothing. It is the zero address — the bit pattern 0x0 — reserved to mean "this pointer does not point at a valid value". Use it to represent the absence of a value and check for it before dereferencing.

var node: *Node = null;   // points nowhere — stored as 0x0

if node != null {
    Process(*node);
}

Why Zero

No real object is ever placed at address 0x0, so the value is unambiguous: a pointer either holds a genuine address or holds null, with no overlap. Comparing against null is therefore just a check for zero:

   Address        Contents
  ┌────────────┬───────────────┐
  │ 0x7ffd00c8 │ 0x00000000    │ ← node : *Node  (null — no target)
  └────────────┴───────────────┘

Dereferencing null

Reading or writing through null means accessing address 0x0, which the operating system leaves unmapped. The result is undefined behavior — in practice a crash (a segmentation fault). Always validate a pointer that may be null before reading through it:

func Length(list: *Node) -> int32 {
    var n = 0;
    var cur = list;
    while cur != null {     // stop before dereferencing null
        n += 1;
        cur = cur.next;
    }
    return n;
}

A null terminator is the conventional way to mark the end of a linked structure — see Self-Referential Structures.

See Also

• Pointers — addresses and how a pointer is stored
• Fields and Members — building linked structures that end in null