MRC vs ARC — what's the difference?

Middle PRO

Essence

Both models are reference counting: an object has a retain count, and when it reaches zero dealloc runs. The difference is who inserts the retain/release/autorelease calls.

• MRC (Manual Reference Counting, before 2011) — the programmer, following the ownership rules: methods named alloc/new/copy/mutableCopy return a +1 object you must release; everything else is autoreleased, and if you want to keep it you retain it.
• ARC (Automatic Reference Counting) — the clang compiler inserts the same calls at compile time by the same rules (including method-name analysis), and the optimizer removes paired retain/release. It is not a garbage collector: no pauses, no graph tracing, nothing happens "by itself" at runtime.

#import <Foundation/Foundation.h>

@interface Person : NSObject
@property (nonatomic, copy) NSString *name;
@end

#if !__has_feature(objc_arc)
// MRC: the setter is written by hand
@implementation Person
- (void)setName:(NSString *)name {
    if (_name != name) {
        [_name release];
        _name = [name copy];
    }
}
- (void)dealloc {
    [_name release];
    [super dealloc];
}
@end
#else
// ARC: the compiler generates the same thing from the copy attribute
@implementation Person
@end
#endif