Question

这个应用程序应该记录自生日（1996年）以来的秒数，但是当我尝试记录NSLog(@"It has been %f seconds since the start of 1996.", seconds);时它会记录：

2014-01-29 11:36:15.756 TimeAfterTime[750:303] It has been (null) seconds since the start of 1996.

我的问题是为什么打印（null）？这是Xcode中的Objective-C命令行工具应用程序。任何建议将不胜感激！感谢

我正在使用的代码：

//  main.m
//  TimeAfterTime
//
//  Created by JP on 1/28/14.
//  Copyright (c) 2014 Big Nerd Ranch. All rights reserved.
//

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[])
{

    @autoreleasepool {

        NSDateComponents *comps = [[NSDateComponents alloc] init];
        [comps setYear:1996];
        [comps setMonth:11];
        [comps setDay:7];
        [comps setHour:13];
        [comps setMinute:23];
        [comps setSecond:0];

        NSCalendar *g = [[[NSCalendar alloc] init] initWithCalendarIdentifier:NSGregorianCalendar];
        NSDate *dateOfBirth = [g dateFromComponents:comps];

        NSDate *now = [[NSDate alloc] init];
        NSLog(@"This NSDate object lives at %p", now);
        NSLog(@"The date is %@", now);

        double seconds = [now timeIntervalSinceDate:dateOfBirth];
        NSLog(@"It has been %f seconds since the start of 1996.", seconds);

    }
    return 0;
}

在控制台中打印的结果：

2014-01-29 11:36:15.752 TimeAfterTime[750:303] This NSDate object lives at 0x100103680
2014-01-29 11:36:15.756 TimeAfterTime[750:303] The date is 2014-01-29 16:36:15 +0000
2014-01-29 11:36:15.756 TimeAfterTime[750:303] It has been (null) seconds since the start of 1996.
Program ended with exit code: 0

Answer 1

NSCalendar *g = [[[NSCalendar alloc] init] initWithCalendarIdentifier:NSGregorianCalendar];

应该是

NSCalendar *g = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

由于初始化错误，g = nil因此dateOfBirth = nil。

实例和日历错误？ Objective-C的

1 个答案: