Simon Willison’s Weblog


Tagged Pointer Strings (2015) (via) Mike Ash digs into a fascinating implementation detail of macOS.

Tagged pointers provide a way to embed a literal value in a pointer reference. Objective-C pointers on macOS are 64 bit, providing plenty of space for representing entire values. If the least significant bit is 1 (the pointer is a 64 bit odd number) then the pointer is "tagged" and represents a value, not a memory reference.

Here's where things get really clever. Storing an integer value up to 60 bits is easy. But what about strings?

There's enough space for three UTF-16 characters, with 12 bits left over. But if the string fits ASCII we can store 7 characters.

Drop everything except a-z A-Z.0-9 and we need 6 bits per character, allowing 10 characters to fit in the pointer.

Apple take this a step further: if the string contains just eilotrm.apdnsIc ufkMShjTRxgC4013 ("b" is apparently uncommon enough to be ignored here) they can store 11 characters in that 60 bits!