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  Objective-C 结构体

  • 结构体

    使用Objective-C数组可以定义变量的类型，这些变量可以容纳相同数据类型的多个数据项，但是结构是Objective-C编程中可用的另一种用户定义的数据类型，它允许您组合不同数据类型的数据项。结构体用于表示记录，假设您想跟踪图书馆中的书籍。您可能需要跟踪每本书的以下属性-
    • 标题
    • 作者
    • 学科
    • 书号

  • 定义结构体

    要定义结构体，必须使用struct语句。struct语句定义了一种新的数据类型，该程序具有多个成员。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，并且每个部件定义是一个正常的变量定义，例如int i; 或float f; 或任何其他有效的变量定义。在结构定义的最后，最后一个分号之前，您可以指定一个或多个结构体变量，但这是可选的。这是您声明Book结构的方式- 
    
struct Books {
   NSString *title;
   NSString *author;
   NSString *subject;
   int   book_id;
} book;

  • 访问结构体成员

    要访问结构的任何成员，我们使用成员访问运算符（.）。成员访问运算符被编码为结构变量名和我们希望访问的结构成员之间的句点。您将使用struct关键字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后，将产生以下结果- 
    
2020-08-08 11:41:09.971 test[4472:8156] Book 1 title : Objective-C Programming
2020-08-08 11:41:09.978 test[4472:8156] Book 1 author : Nuha Ali
2020-08-08 11:41:09.978 test[4472:8156] Book 1 subject : Objective-C Programming Tutorial
2020-08-08 11:41:09.978 test[4472:8156] Book 1 book_id : 6495407
2020-08-08 11:41:09.978 test[4472:8156] Book 2 title : Telecom Billing
2020-08-08 11:41:09.978 test[4472:8156] Book 2 author : Zara Ali
2020-08-08 11:41:09.978 test[4472:8156] Book 2 subject : Telecom Billing Tutorial
2020-08-08 11:41:09.978 test[4472:8156] Book 2 book_id : 6495700

  • 结构作为函数参数

    您可以通过与传递任何其他变量或指针非常相似的方式将结构作为函数参数传递。您将以与上述示例相同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;
}

    
2020-08-08 11:47:19.115 test[9272:5816] Book title : Objective-C Programming
2020-08-08 11:47:19.121 test[9272:5816] Book author : Nuha Ali
2020-08-08 11:47:19.121 test[9272:5816] Book subject : Objective-C Programming Tutorial
2020-08-08 11:47:19.121 test[9272:5816] Book book_id : 6495407
2020-08-08 11:47:19.121 test[9272:5816] Book title : Telecom Billing
2020-08-08 11:47:19.121 test[9272:5816] Book author : Zara Ali
2020-08-08 11:47:19.121 test[9272:5816] Book subject : Telecom Billing Tutorial
2020-08-08 11:47:19.121 test[9272:5816] Book book_id : 6495700

  • 指向结构的指针

    您可以按照与定义任何其他变量的指针非常相似的方式来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。要找到结构变量的地址，请将＆运算符放在结构名称之前，如下所示： 
    
struct_pointer = &Book1;
    要使用指向结构的指针来访问该结构的成员，必须使用->运算符，如下所示： 
    
struct_pointer->title;
    让我们使用结构指针重新编写上面的示例，希望这将使您容易理解概念- 
    
#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后，将产生以下结果- 
    
2020-08-08 11:53:17.372 test[12828:7760] Book title : Objective-C Programming
2020-08-08 11:53:17.379 test[12828:7760] Book author : Nuha Ali
2020-08-08 11:53:17.379 test[12828:7760] Book subject : Objective-C Programming Tutorial
2020-08-08 11:53:17.379 test[12828:7760] Book book_id : 6495407
2020-08-08 11:53:17.379 test[12828:7760] Book title : Telecom Billing
2020-08-08 11:53:17.379 test[12828:7760] Book author : Zara Ali
2020-08-08 11:53:17.379 test[12828:7760] Book subject : Telecom Billing Tutorial
2020-08-08 11:53:17.379 test[12828:7760] Book book_id : 6495700

  • 位域

    位域允许将数据打包在结构中。当内存或数据存储非常宝贵时，这尤其有用。典型例子-
    • 将几个对象打包成一个机器字。例如，可以压缩1位标志。
    • 读取外部文件格式-可以读取非标准文件格式。例如9位整数。
    Objective-C允许我们在结构定义中通过在变量后加上:bit长度来做到这一点。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;
    在这里，packed_struct包含6个成员：四个1位标志f1..f3，一个4位类型和一个9位my_int。如果字段的最大长度小于或等于计算机的整数字长，那么Objective-C会尽可能紧凑地自动打包上述位字段。如果不是这种情况，则某些编译器可能允许字段的内存重叠，而另一些编译器会将下一个字段存储在下一个字中。
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