参见英文答案 > Objective-C and Pointers                                    3个
>             why most of the objects we create in iphone are pointers                                    1个
我是Objective-C的新手,来自Java背景.我刚刚在Objective-C中进行了转换,但我正在使用的书未能解释在转换时使用’*’/指针.以下是他们给我的例子：

myFraction = (Fraction *) fraction;

是不是特定对象的指针,所以他们有自己独特的内存位置？那么为什么我只能在引用类时使用指针？在这种情况下,分数.

谢谢,我希望这是有道理的,我知道这是一个我应该知道和理解的简单问题,但我找不到任何解释.

*符号有多种含义(旁边乘法:)：

>解除引用(跟随)指针.此代码跟随存储在pointerToInt中的指针,然后为其赋值.

(*pointerToInt) = 5;

>声明指针类型.当你写int *它意味着“引用一个整数”.

int x = 5;
int * xPtr = &x

现在,对象是一种结构,但我们只能通过指针操作它们.永远不要直接这基本上意味着,99％的时间当你看到*(并且它不是乘法:)它是第二种情况：类型声明的一部分：

> Nsstring * =指向Nsstring结构的指针(不能单独使用Nsstring)
>分数* =分数结构和分数结构的指针在分数类中描述

所以它不是“指向Fraction类的指针”,而是“指向Fraction类结构的指针”.

我会更进一步,回答你关于两个**的未来问题.您可能会看到通常使用NSError参数,这些参数定义为methodWithError：(NSError **)errorPtr.

短篇小说：int是int *,因为NSError *是NSError **.

长话：如果我们不能直接操作对象(没有指向它们),单个指针就成为声明的标准部分.现在如果我们想要间接访问该对象呢？我们用双指针！对象需要First *,第二个是间接.

NSError *error = nil; // Empty.
NSError **errorPtr = &error; // Reference to our local `error` variable.

[data writetoURL:URL options:kNilOptions error:errorPtr];
// That method uses:   (*errorPtr) = [NSError errorWith...];

NSLog(@"Error: %@",error); // Our local error is no longer empty.

我相信当你来自Java时,指针很奇怪.它们是C的一些遗产,但它们并没有以任何疯狂的方式使用.

我遇到了下一个错误,无法理解如何解决它.

Can’t resolve all parameters for AuthenticationService: ([object Object],?,[object Object])

我已经检查了几乎每个主题,并尝试了多种方法来解决它,但仍然无法在第二天击败它.

我试图像这样在appService中注入第一个authService但是得到了同样的错误

@Inject(forwardRef(() => AuthenticationService)) public authService: AuthenticationService

我检查了所有DI和服务内部的导入顺序,在我看来一切都是正确的

如果有人可以帮我处理它,我很感激.

Angular 4.0.0

AuthService

import { Injectable } from '@angular/core';
import {Http,Headers,Response} from '@angular/http';
import 'rxjs/add/operator/toPromise';
import {Observable} from 'rxjs/Rx';

import {AppServices} from "../../app.services";
import {Router} from "@angular/router";

@Injectable()
export class AuthenticationService {
  public token: any;

  constructor(
    private http: Http,private appService: AppServices,private router: Router
  ) {
    this.token = localStorage.getItem('token');
  }

  login(username: string,password: string): Observable<boolean> {
    let headers = new Headers();
    let body = null;
    headers.append("Authorization",("Basic " + btoa(username + ':' + password)));

    return this.http.post(this.appService.api + '/login',body,{headers: headers})
      .map((response: Response) => {
        let token = response.json() && response.json().token;
        if (token) {
          this.token = token;
          localStorage.setItem('Conform_token',token);
          return true;
        } else {
          return false;
        }
      });
  }

  logout(): void {
    this.token = null;
    localStorage.removeItem('Conform_token');
    this.router.navigate(['/login']);
  }
}

应用服务

import {Injectable} from '@angular/core';
import {Headers,Http,RequestOptions} from '@angular/http';
import {Router} from "@angular/router";
import {AuthenticationService} from "./auth/auth.service";

import 'rxjs/add/operator/toPromise';
import {Observable} from 'rxjs/Rx';

@Injectable()

export class AppServices {

  api = '//endpoint/';

  public options: any;
  constructor(
    private http: Http,private router: Router,public authService: AuthenticationService // doesn't work
  //  @Inject(forwardRef(() => AuthenticationService)) public authService: AuthenticationService // doesn't work either
      ) {
        let head = new Headers({
      'Authorization': 'Bearer ' + this.authService.token,"Content-Type": "application/json; charset=utf8"
    });
    this.options = new RequestOptions({headers: head});
  }

  // ====================
  //    data services
  // ====================

  getData(): Promise<any> {
    return this.http
      .get(this.api + "/data",this.options)
      .toPromise()
      .then(response => response.json() as Array<Object>)
      .catch((err)=>{this.handleError(err);})
  }

应用模块

import { browserModule } from '@angular/platform-browser';
import { browserAnimationsModule } from '@angular/platform-browser/animations';

import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import {BaseRequestOptions,HttpModule} from '@angular/http';

import { MaterialModule} from '@angular/material';
import {FlexLayoutModule} from "@angular/flex-layout";
import 'hammerjs';

import { routing,appRoutingProviders }  from './app.routing';
import { AppServices } from './app.services';
import {AuthGuard} from "./auth/auth.guard";
import {AuthenticationService} from "./auth/auth.service";

import {AppComponent} from './app.component';
import {AuthComponent} from './auth/auth.component';
import {NotFoundComponent} from './404/not-found.component';
import { HomeComponent } from './home/home.component';

@NgModule({
  declarations: [
    AppComponent,AuthComponent,NotFoundComponent,HomeComponent
  ],imports: [
    browserModule,browserAnimationsModule,FormsModule,HttpModule,routing,MaterialModule,FlexLayoutModule
  ],providers: [AppServices,AuthGuard,AuthenticationService],bootstrap: [AppComponent]
})
export class AppModule { }

AppServices和AuthenticationService之间存在循环依赖关系 – 这与Angular使用的构造函数注入无法实现.

你可以使用

export class AuthenticationService {
  public token: any;
  appService: AppServices;
  constructor(
    private http: Http,// private appService: AppServices,injector:Injector;
    private router: Router
  ) {
    setTimeout(() => this.appService = injector.get(AppServices));
    this.token = localStorage.getItem('token');
  }

另见DI with cyclic dependency with custom HTTP and ConfigService

要避免使用setTimeout,您还可以从AppService的构造函数中设置AuthenticationService.appService(或者相反)

@interface Note : NSObject {
    Nsstring *title;
    Nsstring *author;
    BOOL published;
}

@property (nonatomic, copy) Nsstring *title;
@property (nonatomic, copy) Nsstring *author;
@property (nonatomic) BOOL published;

@end

#import "Note.h"

@implementation Note

@synthesize title;
@synthesize author;
@synthesize published;

- (void)dealloc {
    [title release];
    [author release];
    [super dealloc];
}

@end

NSObject <NSCoding> {
    Nsstring *title;
    Nsstring *author;
    dealloc {
    [title release];
    [author release];
    [super dealloc];
}

- (id)initWithCoder:(NSCoder *)decoder {
    if (self = [super init]) {
        self.title = [decoder decodeObjectForKey:@"title"];
        self.author = [decoder decodeObjectForKey:@"author"];
        self.published = [decoder decodeBoolForKey:@"published"];
    }
    return self;
}

- (encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:title forKey:@"time"];
    [encoder encodeObject:author forKey:@"author"];
    [encoder encodeBool:published forKey:@"published"];
}

@end

// Given `notes` contains an array of Note objects
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:notes];
[[NSUserDefaults standardUserDefaults] setobject:data forKey:@"notes"];

NSData *notesData = [[NSUserDefaults standardUserDefaults] objectForKey:@"notes"];
NSArray *notes = [NSKeyedUnarchiver unarchiveObjectWithData:notesData];

根据apple objective-c文档,NSObject使用C指针来跟踪它们.我对iOS很新,并且将NSObjects上的指针操作与C指针进行比较会让人感到困惑.
例如：

Nsstring *string1 = @"This is a string";
Nsstring *string2;
string2 = string 1;

在C中,(如果我错了请纠正我)this = b / w 2指针使它们指向相同的“指针”.这意味着更改string1也应该更改string2.但它似乎没有像这样工作,并且Nsstrings无论如何都是严格不变的,所以这增加了一些混乱.

string1 = @"new string";

如果这些指针像C指针一样操作,那么这不应该改变string2,因为它指向与string1相同的位置.此外,在C中,必须先将指针指定给指针对象才能取消引用.此规则似乎不适用于NSObjects.什么是’@’在做什么？最后,为什么我没有看到NSObjects的解引用发生如下：

*string1 = @"modifying the string"; //shouldn't this be how to access the contents of the pointer string1 if it operates like a c pointer?

有人可以对Objective-C指针下的内容以及它们与C指针的比较和对比有所了解吗？任何帮助将不胜感激.

您将在C中获得与Objective-C中相同的行为：如果您这样做

char *a = "hello";
char *b = a;
b = "world";

然后a不会改变.当对象可变时,对多个指针指向的公共对象所做的更改将变为可见. Nsstring和C字符串文字都不可变,所以让我们构建一个不同的例子：

NSMutableString *string1 = [NSMutableString string:@"This is a string"];
NSMutableString *string2;
string2 = string1;
NSLog(string1);
NSLog(string2);
[string1 appendString:@" (a very long one!)"];
NSLog(string1);
NSLog(string2);

现在string2“看到”对string1所做的更改,因为该对象是可变的.

C中的类似示例如下所示：

char[] a = "hello";
char b = a;
printf("%s %s\n",a,b);
strcpy(b,"world");
printf("%s %s\n",b);

我最近发现了 eero programming lanugage
我发现它的概念非常有趣.它似乎主要是语法糖,看起来很可读.

虽然它不直接编译到Objective-C,但它声称生成与Objective-C相同的二进制代码

Eero compiles down to the same binary code as Objective-C

Eero offers excellent,nearly seamless interoperability with
Objective-C,C,and C++.

我发现这种方法非常有趣,我想知道是否有类似的编程语言和项目提供与Objective-C和iOS的大腿集成.
我正在为Objective-C寻找类似Coffeescript的东西.

虽然使用eero的主要方法是将其编译为本机代码,但它也支持源到源的转换(从eero到标准的Objective-C/C++).有关详细信息,请参阅 https://github.com/eerolanguage/eero/wiki/Translator.

它确实需要记录在一个更明显的地方……