C++ 语言 typedef 关键字

C++ 语言 typedef 关键字

1. typedef in C++

typedef keyword is used to assign a new name to any existing data-type.
typedef 关键字用于为任何现有数据类型分配新名称。

For example, if we want to declare some variables of type unsigned int, we have to write unsigned int in a program and it can be quite hectic for some of us. So, we can assign a new name of our choice for unsigned int using typedef which can be used anytime we want to use unsigned int in a program.
如果要声明一些类型为 unsigned int 的变量，则必须在程序中编写 unsigned int。可以使用 typedef 为unsigned int 分配一个新的名称，该名称可以在我们想在程序中使用 unsigned int 的任何时候使用。

hectic [ˈhektɪk]：adj. 紧张忙碌的，肺病的，脸上发红的，狂热的 n. 脸红，患肺结核

Following is the syntax of typedef (以下是 typedef 的语法)

typedef current_name new_name;

Now, suppose we want to declare two variables of type unsigned int. Instead of writing unsigned int again and again, let’s use a new name uint in its place using typedef as follows:
假设要声明两个类型为 unsigned int 的变量，让我们使用 typedef 这样的新名称 uint 如下：

typedef unsigned int uint;
uint i, j;

Now, we can write uint in the whole program instead of unsigned int. The above code is the same as writing:
现在可以在整个程序中编写 uint 而不是 unsigned int。上面的代码与下面编写的相同：

unsigned int i, j;

//============================================================================
// Name        : std::string::typedef
// Author      : Yongqiang Cheng
// Version     : Version 1.0.0
// Copyright   : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>

int main() {
	typedef unsigned int ui;

	ui i = 5, j = 8;
	std::cout << "i = " << i << std::endl;
	std::cout << "j = " << j << std::endl;

	return 0;
}

i = 5
j = 8
请按任意键继续. . .

Thus, we can assign a new name to any data type.
我们可以为任何数据类型分配一个新名称。

Similarly, we can also use typedef to assign a new name to structure which is a user-defined datatype as follows:
我们也可以使用 typedef 为结构分配一个新名称，该结构是用户定义的数据类型，如下所示：

typedef struct structure_name
{
    data-type member-1;
    data-type member-2;
    data-type member-3;
    data-type member-4;
} type_name;

特别注意：structure_name 和 type_name 概念不一样。

Now, while declaring variables of this structure type, we can write type_name in place of struct structure_name in the whole program.
在声明这种结构类型的变量时，我们可以在整个程序中写成 type_name 代替 struct structure_name。

//============================================================================
// Name        : std::string::typedef
// Author      : Yongqiang Cheng
// Version     : Version 1.0.0
// Copyright   : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================

#ifndef _CRT_SECURE_NO_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#endif

#include <iostream>
#include <string>

using namespace std;

typedef struct student {
	int index;
	char name[30];
	int phone_number;
} struct_type;

int main() {
	struct_type p1, p2, p3;

	p1.index = 1;
	strcpy(p1.name, "Cheng");
	p1.phone_number = 258;

	p2.index = 2;
	strcpy(p2.name, "Yong");
	p2.phone_number = 369;

	p3.index = 3;
	strcpy(p3.name, "Qiang");
	p3.phone_number = 147;

	cout << "First Student" << endl;
	cout << "index : " << p1.index << endl;
	cout << "name : " << p1.name << endl;
	cout << "phone number : " << p1.phone_number << endl;

	cout << "\nSecond Student" << endl;
	cout << "index : " << p2.index << endl;
	cout << "name : " << p2.name << endl;
	cout << "phone number : " << p2.phone_number << endl;

	cout << "\nThird Student" << endl;
	cout << "index : " << p3.index << endl;
	cout << "name : " << p3.name << endl;
	cout << "phone number : " << p3.phone_number << endl;

	return 0;
}

First Student
index : 1
name : Cheng
phone number : 258

Second Student
index : 2
name : Yong
phone number : 369

Third Student
index : 3
name : Qiang
phone number : 147
请按任意键继续. . .

Here, the whole example is the same as we did in structure, the only difference is that we wrote struct_type in place of struct student i.e. we used the new type (named struct_type) to declare the variables of this structure type (named student).
使用 struct_type 代替 struct student，即使用了新的类型 (名为 struct_type) 来声明此结构的变量类型 (命名学生)。

We can also use typedef with union. For this, everything will be same as that of the structure with the keyword union in the place of struct.
也可以将 typedef 和 union 一起使用，所有内容都与使用关键字 union 代替 struct 的结构相同。

2. typedef 关键字

//============================================================================
// Name        : Yongqiang Cheng
// Author      : Yongqiang Cheng
// Version     : Version 1.0.0
// Copyright   : Copyright (c) 2020 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================
#include <iostream>

int main()
{
	typedef struct Books
	{
		char title[49];
		char author[19];
		int number;
	} Books;

	Books book1, book2;

	return 0;
}

typedef 关键字定义非结构类型：

//============================================================================
// Name        : Yongqiang Cheng
// Author      : Yongqiang Cheng
// Version     : Version 1.0.0
// Copyright   : Copyright (c) 2020 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================
#include <iostream>

int main()
{
	typedef int *pint32;
	pint32 x, y;

	int *m, n;

	return 0;
}

x、y 和 m 都是指向 int 的指针 (int *)，n 是 int 数据。

//============================================================================
// Name        : Yongqiang Cheng
// Author      : Yongqiang Cheng
// Version     : Version 1.0.0
// Copyright   : Copyright (c) 2020 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================

#ifndef _CRT_SECURE_NO_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#endif

#include <iostream>
using std::string;

int main()
{
	int yq = 9;
	const int cyq = 6;

	typedef int *pint;
	const pint x(&yq), y(&yq);

	return 0;
}

References

https://www.codesdope.com/cpp-typedef/