Reference counting

Junior PRO

Essence

Reference counting (retain count) is the mechanism memory management in Objective-C and Swift is built on. Every heap object has a count of strong owners:

• creating the object (alloc/init, Class()) — count = 1;
• a new strong owner (retain, assignment to a strong variable) — +1;
• an owner lets go (release, variable leaves scope) — −1;
• autorelease — "−1 later", when the autorelease pool drains;
• the count hits 0 — dealloc/deinit runs and the memory is freed.

Under MRC those calls were written by hand; under ARC the compiler inserts them, but the counters themselves exist and are incremented at run time (atomically).