iOS单例的正确创建

#import "CoolObject.h"

@implementation CoolObject
+(id)shareInstance {
    static CoolObject *instance = nil;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
//        必须使用super防止循环调用 self和CoolObject一样都会导致循环调用
        instance = [[super allocWithZone:NULL] init];
    });
    return instance;
}

/// 防止有人使用allocWithZone创建对象
/// @param zone 一般传null
+(instancetype)allocWithZone:(struct _NSZone *)zone {
    return [self shareInstance];
}

/// 防止有人使用copy 单例对象
/// @param zone
-(id)copyWithZone:(NSZone *)zone {
    return self;
}

@end

CoolObject *instance = [CoolObject shareInstance];
CoolObject *copyInstance = [instance copy];
CoolObject *zoneInstance = [[CoolObject allocWithZone:nil] init];
NSLog(@"instance: %p %p %p", instance, copyInstance, zoneInstance);
//输出
instance: 0x600000a38230 0x600000a38230 0x600000a38230