Cprogramming 简明教程

Typedef in C

C typedef

C 语言提供一个名为 typedef 的关键字，用于为现有 data type 设置备用名称。C 中的 typedef 关键字非常适用于为内置数据类型以及派生数据类型（例如结构体、联合或指针）分配一个方便的别名。

The C programming language provides a keyword called typedef to set an alternate name to an existing data type. The typedef keyword in C is very useful in assigning a convenient alias to a built-in data type as well as any derived data type such as a struct, a union or a pointer.

有时，在每次声明变量时，使用名称较长的数据类型（例如“ struct structname ”或“ unsigned int ”）会变得很笨重。在这些情况下，我们可以分配一个便捷的快捷方式来使代码更具可读性。

Sometimes it becomes clumsy to use a data type with a longer name (such as "struct structname" or "unsigned int") every time a variable is declared. In such cases, we can assign a handy shortcut to make the code more readable.

typedef Syntax

一般来说， typedef 关键字按如下方式使用：

In general, the typedef keyword is used as follows −

typedef existing_type new_type;

typedef Examples

在本节中，让我们了解 typedef 的一些用例。

In this chapter, let us have a look at some of the use-cases of typedef.

Example 1

在 C 语言中，关键字“ unsigned ”用于声明无符号整数 variables ，该整数只能存储非负值。

In C language, the keyword "unsigned" is used to declare unsigned integer variables that can store only non-negative values.

C 还有一个名为“ short ”的关键字，该关键字声明占用 2 个字节内存的整数数据类型。如果你想要声明一个为 short 且只能具有非负值的变量，那么可以组合这两个关键字（unsigned 和 short）：

C also has a keyword called "short" that declares an integer data type that occupies 2 bytes of memory. If you want to declare a variable that is short and can have only non-negative values, then you can combine both these keywords (unsigned and short):

short unsigned int x;

如果要声明许多这种类型的变量，则每次使用这三个关键字都会非常不方便。相反，可以使用 typedef 关键字定义 alias 或快捷方式，如下所示−

If there are going to be many variables to be declared of this type, it will not be very convenient to use these three keywords every time. Instead, you can define an alias or a shortcut with the typedef keyword as follows −

typedef short unsigned int USHORT;

这会通知编译器，标识符 USHORT 对应于“ short unsigned int ”类型。此后，可以在变量声明语句中使用 USHORT −

This tells the compiler that the identifier USHORT corresponds to "short unsigned int" type. Hereafter, you can use USHORT in the variable declaration statement −

USHORT x;

Example 2

C 还有一个 static 关键字，表示此类变量仅初始化一次。关键字“ long ”在 64 位系统上分配 8 个字节来存储整数。我们可以声明这种类型的变量，如下所示：

C also has the keyword static to indicate that such a variable is initialized only once. The keyword "long" allocates 8 bytes to store an integer on a 64-bit system. We can declare a variable of this type as follows −

static unsigned long x;

然而，我们不能在“typedef”语句中使用关键字“static”，不过，可以使用 typedef 为这种类型的声明分配一个快捷方式别名−

However, we can’t use the keyword "static" in a "typedef" statement, however we can use typedef to assign a shortcut alias to this type of declaration −

typedef signed long SLONG;
static SLONG x;

Note : 根据惯例，别名名称以大写字母表示，只是为了区分内置类型和使用的别名。

Note: As a convention, the alias names are expressed in uppercase, just to differentiate between the built-in type and the aliases used.

Example 3

以下示例演示如何在 C 程序中使用别名名称−

The following example demonstrates how you can use alias names in a C program −

#include <stdio.h>

int main() {
   typedef short unsigned int USHORT;
   typedef signed long int SLONG;

   static SLONG x = 100;
   USHORT y = 200;

   printf("Size of SLONG: %d \nSize of USHORT: %d", sizeof(SLONG), sizeof(USHORT));

   return 0;
}

Output

运行此代码时，将产生以下输出−

When you run this code, it will produce the following ouput −

Size of SLONG: 8
Size of USHORT: 2

Defining a Structure using Typedef

通常，我们需要通过在声明语句中添加 struct_type 的名称前缀来声明结构体变量，如下所示−

Normally, we need to declare a struct variable by prefixing the name of struct_type in the declaration statement as −

struct struct_type var;

如果以这种方式编写类型名称会感觉很麻烦，则可以使用 typedef 来分配别名−

If writing the type name in this manner feels cumbersome, then you can use typedef to assign an alias −

typedef struct struct_type ALIAS;

Example

在这个示例中，我们定义了一个 structure 类型，然后使用 typedef 关键字为它设置一个别名 −

In this example, we define a structure type and then use the typedef keyword to set an alias for it −

#include <stdio.h>

int main() {

   typedef unsigned long int ULONG;
   typedef short int SHORT;

   struct mystruct {
      ULONG a;
      SHORT b;
   };

   typedef struct mystruct STR;
   STR s1 = {10, 20};
   printf("%ld %u", s1.a, s1.b);

   return 0;
}

运行代码并检查其输出：

Run the code and check its output −

10 20

有一个 alternate approach 使用 typedef 关键字。我们可以将其组合在结构定义中，如下所示 −

There is an alternate approach to use the typedef keyword. We can combine it in the structure definition itself, as given below −

typedef struct mystruct {
   ULONG a;
   SHORT b;
} STR;

STR s1 = {10, 20};

Typedef for Struct Pointer

typedef 关键字也可用于为任意 pointer 类型分配一个新标识符。通常，我们这样声明一个指针变量 −

The typedef keyword may also be used to assign a new identifier to any pointer type. Normally, we declare a pointer variable as follows −

struct mystruct * x;

相反，我们可以这样使用 typedef 关键字 −

Instead, we can use the typedef keyword as follows −

typedef struct mystruct {
   ULONG a;
   SHORT b;
} STR;

typedef STR * strptr;

它允许你以一个更简洁的方式来声明这个类型的指针 −

It allows you to declare a pointer of this type in a much more concise way −

strptr ptr;

然后我们可以为对应的 struct variable to the pointer 分配地址。

We can then assign the address of a corresponding struct variable to the pointer.

Example

下面的示例展示了如何使用 typedef 来创建一个结构指针 −

The following example shows how you can use typedef to create a struct prointer −

#include <stdio.h>

int main() {

   typedef unsigned long int ULONG;
   typedef short int SHORT;

   typedef struct mystruct {
      ULONG a;
      SHORT b;
   } STR;

   STR s1 = {10, 20};
   typedef STR * strptr;
   strptr ptr = &s1;

   printf("%d %d \n", s1.a, s1.b);
   printf("%d %d", ptr->a, ptr->b);

   return 0;
}

当你运行这段代码时，它将产生以下输出：

When you run this code, it will produce the following output −

10 20
10 20

Typedef for Union

我们可以使用 typedef 关键字为任意 union 类型分配一个快捷别名。

We can use the typedef keyword to assign a shortcut alias to any union type.

Example

下面的示例展示了如何在创建联合中使用 typedef −

The following example illustrates how you can use typedef in creating unions −

#include <stdio.h>

int main() {

   typedef unsigned long int ULONG;
   typedef short int SHORT;

   typedef union myunion {
      char a;
      int b;
      double c;
   } UNTYPE;

   UNTYPE u1;
   u1.c = 65.50;

   typedef UNTYPE * UNPTR;
   UNPTR ptr = &u1;

   printf("a:%c b: %d c: %lf \n", u1.a, u1.b, u1.c);
   printf("a:%c b: %d c: %lf \n", ptr->a, ptr->b, ptr->c);

   return 0;
}

运行代码并检查其输出：

Run the code and check its output −

a: b: 0 c: 65.500000
a: b: 0 c: 65.500000

typedef vs

typedef 关键字经常与 #define 指令混淆。在 C 语言中， #define 是一个 preprocessor directive 。它是定义常量的有效方法。

The typedef keyword is often confused with the #define directive. In C language, #define is a preprocessor directive. It is an effective method to define a constant.

使用 #define 的 syntax 如下 −

The syntax of using #define is as follows −

#define name value

对于 example −

For example −

#define PI 3.14159265359

#define 语句也可以用于定义宏 −

The #define statement can also be used to define a macro −

#define SQUARE(x) x*x

一个宏的工作方式像一个函数。但是，在其被调用的时候，该值会在预处理程序级别被替换。

A macro works like a function. However, the value is substituted at the preprocessor level when called.

printf("%d", SQUARE(5));

#define 是一个预处理程序指令，而 typedef 会在编译的时候被求值。

#define is a preprocessor directive, while typedef is evaluated at the time of compilation.

  1. typedef is limited to giving symbolic names to types only. #define can be used to define alias for values as well. For example, you can define "1" as "ONE".

  2. typedef interpretation is performed by the compiler, whereas #define statements are processed by the pre-processor.

Example

在下面的代码中，我们使用了这两个特性 ( typedef 和 #define ) −

In the following code, we use both these features (typedef and #define) −

#include <stdio.h>
#define MAX 10

int main() {

   typedef unsigned long int ULONG;
   typedef short int SHORT;

   typedef struct employee {
      char name[MAX];
      int age;
   } EMP;

   EMP e1 = {"Krishna", 25};
   printf("Name: %s \nAge: %d", e1.name, e1.age);

   return 0;
}

当你运行这段代码时，它将产生以下输出：

When you run this code, it will produce the following output −

Name: Krishna
Age: 25