null Versus undefined

Zig has both null and undefined, and they are not the same. Mixing them up is a classic source of confusing bugs.

null is a real, defined state: it says this optional intentionally holds nothing. It is a meaningful, checkable value.

undefined means the storage exists but its contents are garbage. You are promising to write a real value before you read it.

You can assign null only to optional types. A plain i32 will never accept it, because absence is not part of its type.

By contrast, almost any variable can start as undefined to reserve memory now and fill it in a moment later.

Checking an optional against null is perfectly safe and expected; that is exactly how you detect an absent value.

Reading a value that is still undefined is illegal behavior. Safe builds even fill it with 0xAA bytes to expose the mistake.

Think of undefined as a contract: you swear to assign before use. Break it and you read meaningless, unpredictable data.

A frequent idiom is declaring a buffer as undefined, then immediately filling it from a read or a format call.

Use null to model genuine optionality. Use undefined only to skip a redundant initialization you will overwrite right away.

null answers is there a value, while undefined answers has the memory been written yet. Keeping them separate keeps you safe.

You declare var buf: [8]u8 = undefined; and read buf[0] before writing it. What is that?

You separated null (a real absent value for optionals) from undefined (uninitialized memory you must write first). Two ideas, never confused. 🧠