SingleClass.m

#import <Foundation/Foundation.h>
@class SingleClass;

static SingleClass *instance = nil;

@interface SingleClass : NSObject <NSCopying>
+ (instancetype)shareInstance;
@end

@implementation SingleClass
//类方法，获取单例对象
+ (instancetype)shareInstance{
    if (!instance) {
        instance = [[self alloc] init];
    }
    return instance;
}
//类方法，调用对象的alloc方法时返回已存在的单例
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (!instance) {
        instance = [super allocWithZone:zone];
    }
    return instance;
}
//copy时，返回已存在的单例
- (id)copyWithZone:(NSZone *)zone{
    return instance;
}
//copy时，返回已存在的单例
- (id)copy{
    return instance;
}
@end

main.m

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        SingleClass *s1 = [SingleClass shareInstance];
        SingleClass *s2 = [SingleClass shareInstance];
        SingleClass *s3 = [[SingleClass alloc] init];
        SingleClass *s4 = [s1 copy];
        
        NSLog(@"%p",s1);
        NSLog(@"%p",s2);
        NSLog(@"%p",s3);
        NSLog(@"%p",s4);
    }
    return 0;
}

结果：

2015-03-22 22:24:21.201 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0