iOS错误：’xxxx’没有可见的@interface声明选择器’alloc’

//TextValidator.h
#import <Foundation/Foundation.h>

@interface TextValidator : NSObject
- (BOOL) isValidPassword:(Nsstring *)checkPassword;
- (BOOL) isValidEmail:(Nsstring *)checkString;
- (BOOL) isEmpty:(Nsstring *)checkString;
@end 


//  TextValidator.m
#import "TextValidator.h"

@implementation TextValidator

- (BOOL) isEmpty:(Nsstring *)checkString
{
    return YES;
}

- (BOOL) isValidPassword:(Nsstring *)checkPassword
{
return YES;
}

- (BOOL) isValidEmail:(Nsstring *)checkString
{
return YES;
}

@end

这是我尝试在ViewController.m中初始化TextValidator类的方法：

//ViewController.h
#import <Foundation/Foundation.h>

@interface SignUpViewController : UIViewController <UITextFieldDelegate>
@end

//ViewController.m
#import "SignUpViewController.h"
#import "TextValidator.h"

@interface SignUpViewController ()

@property TextValidator *myValidator;

@end

@implementation SignUpViewController

- (void)viewDidLoad
{
    [[self.myValidator alloc] init]; //iOS error: No visible @interface for 'TextValidator' declares the selector 'alloc'*
    [super viewDidLoad];
}
@end

当我尝试编译代码时,我收到以下错误：

No visible @interface for 'TextValidator' declares the selector 'alloc'.

TextValidator类继承自NSObject,据我所知,init和alloc函数已在基类中定义.那么为什么程序会出现这样的错误呢？

请注意,我已经检查了this主题,但它对我不起作用.