单例模式－ios

#import <Foundation/Foundation.h>

@interface UserContext : NSObject <NSCopying>

@property (nonatomic,copy) NSString *name;
@property (nonatomic,copy) NSString *emial;

+(id) shareUsercontext;

@end

#import "UserContext.h"

static UserContext *segtonInstance=nil;

@implementation UserContext

+(id) shareUsercontext{

    @synchronized(self){

    if(segtonInstance==nil){
        segtonInstance=[[[self class] alloc] init];
        }

    }
    return segtonInstance;
}

+(id) allocWithZone:(struct _NSZone *)zone{
    if (segtonInstance==nil) {
        segtonInstance=[super allocWithZone:zone];
    }
    return segtonInstance;
}

- (id)copyWithZone:(NSZone *)zone{

    return segtonInstance;

}

@end

/**单例模式***/
        UserContext *usercontext=[UserContext shareUsercontext];
        UserContext *usercontext2=[UserContext shareUsercontext];
        UserContext *usercontext3=[[UserContext alloc] init];
        UserContext *usercontext4=[usercontext copy];
        NSLog(@"%p",usercontext);
        NSLog(@"%p",usercontext2);
        NSLog(@"%p",usercontext3);
        NSLog(@"%p",usercontext4);