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Interesting notes on Objective-C "tagged pointers" in Lion: Some objects may not be actual objects! /via @kimon

CFNumber (and, consequently, NSNumber) takes advantage of tagged pointers for integers — if an integer can fit in the payload of a tagged pointer then no actual CFNumber is created. Instead, the memory address represents the integer number itself according to the following layout…

"Every tagged pointer has its lowest bit set, hence tagged pointers are odd integers" Strikes me as a really bad idea.