单例模式（oc）

//主函数 main.m
#import <Foundation/Foundation.h>
#import "Singleton.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
    
        Singleton *sing1=
        [Singleton shareInstance];
        Singleton *sing2=[Singleton shareInstance];
       
        Singleton *sing3=[Singleton new];
        Singleton *sing4=[[Singleton alloc]init];
        Singleton *sing5=[sing4 copy];
       
        NSLog(@"%@",sing1);
        NSLog(@"%@",sing2);
        NSLog(@"%@",sing3);
        NSLog(@"%@",sing4);
        NSLog(@"%@",sing5);
       
    }
    return 0;
}

//Singleton.h文件
#import <Foundation/Foundation.h>

@interface Singleton : NSObject<NSCopying>
+(Singleton *)shareInstance;
@end


//Singleton.m文件
#import "Singleton.h"

@implementation Singleton
static Singleton *singleton;
//单例方法
+(Singleton *)shareInstance{
    if (singleton==nil) {
        singleton=[[Singleton alloc]init];
    }
    return singleton;

}
//单例方法  初始化
+(instancetype)allocWithZone:(struct _NSZone *)zone{
//懒加载
    if (singleton==nil) {
        singleton=[super allocWithZone:zone];
    }
    return singleton;
}
//单例方法 复制
-(id)copyWithZone:(NSZone *)zone{
    return self;
}
@end