Objective-C - 结构(C - Structures)

Objective-C数组允许您定义可以包含多个相同类型的数据项的变量类型，但structure是Objective-C编程中可用的另一个用户定义数据类型，它允许您组合不同类型的数据项。

结构用于表示记录，假设您想要在库中跟踪您的书籍。您可能希望跟踪每本书的以下属性 -

Title
Author
Subject
书名

定义一个结构 (Defining a Structure)

要定义结构，必须使用struct语句。 struct语句定义一个新的数据类型，为您的程序提供多个成员。 struct语句的格式如下所示 -

struct [structure tag] {
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];

structure tag是可选的，每个成员定义是一个普通的变量定义，例如int i; 或浮动f; 或任何其他有效的变量定义。在结构定义的最后，在最后一个分号之前，您可以指定一个或多个结构变量，但它是可选的。以下是您声明Book结构的方式 -

struct Books {
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
} book;

访问结构成员 (Accessing Structure Members)

要访问结构的任何成员，我们使用member access operator (.) 。成员访问运算符被编码为结构变量名称和我们希望访问的结构成员之间的句点。您可以使用struct关键字来定义结构类型的变量。以下是解释结构用法的示例 -

#import <Foundation/Foundation.h>
struct Books {
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};
int main() {
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;
   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
   /* print Book1 info */
   NSLog(@"Book 1 title : %@\n", Book1.title);
   NSLog(@"Book 1 author : %@\n", Book1.author);
   NSLog(@"Book 1 subject : %@\n", Book1.subject);
   NSLog(@"Book 1 book_id : %d\n", Book1.book_id);
   /* print Book2 info */
   NSLog(@"Book 2 title : %@\n", Book2.title);
   NSLog(@"Book 2 author : %@\n", Book2.author);
   NSLog(@"Book 2 subject : %@\n", Book2.subject);
   NSLog(@"Book 2 book_id : %d\n", Book2.book_id);
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-14 04:20:07.947 demo[20591] Book 1 title : Objective-C Programming
2013-09-14 04:20:07.947 demo[20591] Book 1 author : Nuha Ali
2013-09-14 04:20:07.947 demo[20591] Book 1 subject : Objective-C Programming Tutorial
2013-09-14 04:20:07.947 demo[20591] Book 1 book_id : 6495407
2013-09-14 04:20:07.947 demo[20591] Book 2 title : Telecom Billing
2013-09-14 04:20:07.947 demo[20591] Book 2 author : Zara Ali
2013-09-14 04:20:07.947 demo[20591] Book 2 subject : Telecom Billing Tutorial
2013-09-14 04:20:07.947 demo[20591] Book 2 book_id : 6495700

作为函数参数的结构 (Structures as Function Arguments)

您可以将结构作为函数参数传递，与传递任何其他变量或指针的方式非常相似。您将以与上面示例中访问过的方式类似的方式访问结构变量 -

#import <Foundation/Foundation.h>
struct Books {
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};
@interface SampleClass:NSObject
/* function declaration */
- (void) printBook:( struct Books) book ;
@end
@implementation SampleClass 
- (void) printBook:( struct Books) book {
   NSLog(@"Book title : %@\n", book.title);
   NSLog(@"Book author : %@\n", book.author);
   NSLog(@"Book subject : %@\n", book.subject);
   NSLog(@"Book book_id : %d\n", book.book_id);
}
@end
int main() {
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;
   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
   SampleClass *sampleClass = [[SampleClass alloc]init];
   /* print Book1 info */
   [sampleClass printBook: Book1];
   /* Print Book2 info */
   [sampleClass printBook: Book2];
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-14 04:34:45.725 demo[8060] Book title : Objective-C Programming
2013-09-14 04:34:45.725 demo[8060] Book author : Nuha Ali
2013-09-14 04:34:45.725 demo[8060] Book subject : Objective-C Programming Tutorial
2013-09-14 04:34:45.725 demo[8060] Book book_id : 6495407
2013-09-14 04:34:45.725 demo[8060] Book title : Telecom Billing
2013-09-14 04:34:45.725 demo[8060] Book author : Zara Ali
2013-09-14 04:34:45.725 demo[8060] Book subject : Telecom Billing Tutorial
2013-09-14 04:34:45.725 demo[8060] Book book_id : 6495700

指向结构的指针

您可以按照与定义指向任何其他变量的指针非常相似的方式定义指向结构的指针，如下所示 -

struct Books *struct_pointer;

现在，您可以将结构变量的地址存储在上面定义的指针变量中。要查找结构变量的地址，请将＆运算符放在结构名称之前，如下所示 -

struct_pointer = &Book1;

要使用指向该结构的指针访问结构的成员，必须使用 - >运算符，如下所示 -

struct_pointer->title;

让我们用结构指针重新编写上面的例子，希望这对你来说很容易理解 -

#import <Foundation/Foundation.h>
struct Books {
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
};
@interface SampleClass:NSObject
/* function declaration */
- (void) printBook:( struct Books *) book ;
@end
@implementation SampleClass 
- (void) printBook:( struct Books *) book {
   NSLog(@"Book title : %@\n", book->title);
   NSLog(@"Book author : %@\n", book->author);
   NSLog(@"Book subject : %@\n", book->subject);
   NSLog(@"Book book_id : %d\n", book->book_id);
}
@end
int main() {
   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */
   /* book 1 specification */
   Book1.title = @"Objective-C Programming";
   Book1.author = @"Nuha Ali"; 
   Book1.subject = @"Objective-C Programming Tutorial";
   Book1.book_id = 6495407;
   /* book 2 specification */
   Book2.title = @"Telecom Billing";
   Book2.author = @"Zara Ali";
   Book2.subject = @"Telecom Billing Tutorial";
   Book2.book_id = 6495700;
   SampleClass *sampleClass = [[SampleClass alloc]init];
   /* print Book1 info by passing address of Book1 */
   [sampleClass printBook:&Book1];
   /* print Book2 info by passing address of Book2 */
   [sampleClass printBook:&Book2];
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-14 04:38:13.942 demo[20745] Book title : Objective-C Programming
2013-09-14 04:38:13.942 demo[20745] Book author : Nuha Ali
2013-09-14 04:38:13.942 demo[20745] Book subject : Objective-C Programming Tutorial
2013-09-14 04:38:13.942 demo[20745] Book book_id : 6495407
2013-09-14 04:38:13.942 demo[20745] Book title : Telecom Billing
2013-09-14 04:38:13.942 demo[20745] Book author : Zara Ali
2013-09-14 04:38:13.942 demo[20745] Book subject : Telecom Billing Tutorial
2013-09-14 04:38:13.942 demo[20745] Book book_id : 6495700

位域

位字段允许在结构中打包数据。当内存或数据存储非常宝贵时，这尤其有用。典型例子 -

将多个对象打包成机器字。例如，可以压缩1位标志。
读取外部文件格式 - 可以读入非标准文件格式。例如9位整数。

Objective-C允许我们通过在变量之后放置:位长度来在结构定义中执行此操作。例如 -

struct packed_struct {
   unsigned int f1:1;
   unsigned int f2:1;
   unsigned int f3:1;
   unsigned int f4:1;
   unsigned int type:4;
   unsigned int my_int:9;
} pack;

这里，packed_struct包含6个成员:四个1位标志f1..f3,4位类型和9位my_int。

Objective-C尽可能紧凑地自动打包上述位字段，前提是字段的最大长度小于或等于计算机的整数字长。如果不是这种情况，那么一些编译器可能允许字段的存储器重叠，而其他编译器将下一个字段存储在下一个字中。

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Objective-C - 教程

Objective-C - 概述

Objective-C - 环境设置(C - Environment Setup)

Objective-C - 程序结构

Objective-C - Basic 语法

Objective-C - 数据类型

Objective-C - 变量

Objective-C - 常量

Objective-C - 运算符

Objective-C - 循环

Objective-C - 决策(C - Decision Making)

Objective-C - 功能(C - Functions)

Objective-C - 块(C - Blocks)

Objective-C - 数字(C - Numbers)

Objective-C - 数组(C - Arrays)

Objective-C - 指针(C - Pointers)

Objective-C - 字符串(C - Strings)