Objective-C(+ARC)で弱参照キャッシュ

QzCacheMap.h

#import <Foundation/Foundation.h>

// QzCacheMapにキャッシュさせる為には、これを継承させる
@interface QzCacheEntry : NSObject
@end

@interface QzCacheMap : NSObject

- (id)initWithCapacity:(size_t)capacity;

// キャッシュするオブジェクトをキーに割り当てる
- (void)put:(QzCacheEntry*)entry
     forKey:(NSString*)key;

// キーに対応したキャッシュ済みのオブジェクトを取得
- (id)get:(NSString*)key;

// 削除用
- (BOOL)remove:(__unsafe_unretained QzCacheEntry*)entry;

@end

QzCacheMap.m

#import "QzCacheMap.h"

@interface QzCacheEntry ()
{
@package
	QzCacheMap *_cacheMap;
	NSString *_assigned;
}
@end

@implementation QzCacheMap
{
	NSMutableDictionary *_dic;
	size_t _capacity;
}

- (id)initWithCapacity:(size_t)capacity
{
	if((self = [self init])) {
		self->_dic      = [NSMutableDictionary dictionaryWithCapacity:capacity];
		self->_capacity = capacity;
	}
	return self;
}

- (void)put:(QzCacheEntry*)entry
     forKey:(NSString*)key
{
	[_dic setObject:[NSNumber numberWithUnsignedLong:(uintptr_t)entry]
                 forKey:key];
	entry->_cacheMap = self;
	entry->_assigned = key;
}

- (id)get:(NSString*)key
{
	NSNumber *box = [_dic objectForKey:key];
	if(!box) {
		return nil;
	}
	return (__bridge id) (void*) [box unsignedLongValue];
}

- (BOOL)remove:(__unsafe_unretained QzCacheEntry*)entry
{
	// 別の人が管理してるキャッシュ
	if(entry->_cacheMap != self) {
		return NO;
	}
	NSNumber *box = [_dic objectForKey:entry->_assigned];
	// エントリーが存在しない
	if(!box) {
		return NO;
	}
	// キーに割り当てられているのは別のキャッシュ
	if((uintptr_t)entry != [box unsignedLongValue]) {
		return NO;
	}
	[_dic removeObjectForKey:entry->_assigned];
	return YES;
}
@end

@implementation QzCacheEntry

- (void)dealloc
{
	[_cacheMap remove:self];
}
@end