Objective C中没有声明的标示符

实在是不知道该怎么绕过这个错误，总是说：use of undeclared identifier 'ageBy'。请高手帮我指点一下吧，先看看我的代码：

Person.h

@interface Person : NSObject
{
 int _age;
 int _years;
 NSString *_name;
 NSString *_job;

} 

-(void)setAge:(int)age;
-(int)age;

-(void)setName:(NSString *)name;
-(NSString *)name;

-(void)setJob:(NSString *)job;
-(NSString *)job;

-(NSString *)summaryString;

-(void)ageBy:(int)years;


@end

Person.m

#import "Person.h"
@implementation Person

-(void)setAge:(int)age{
  _age = age;
}
-(int)age{
  return _age;
}
-(void)setName:(NSString *)name{
  _name = name;
}
-(NSString *)name{
  return _name; }

-(void)setJob:(NSString *)job{
  _job = job;
}
-(NSString *)job{
  return _job;
}

-(NSString *)summaryString{
  return [NSString stringWithFormat:@"The Person %@ is %d years old and is a  %@",_name,_age,_job];

-(void)ageBy:(int)years{
  _years = years;
  _age = years + _age;

 }

 } 
 @end

你的ageBy是在summaryString中定义的，应该移动{}括号到@end之前，这样它就在
-(void)ageBy:(int)years中了，

-(NSString *)summaryString{
  return [NSString stringWithFormat:@"The Person %@ is %d years old and is a  %@",_name,_age,_job];
 } 

-(void)ageBy:(int)years{
  _years = years;
  _age = years + _age;

 }