Custom Types - Enums, Unions

目录

一、枚举

1、定义

 2、Features and Benefits

①Values ​​in enumeration types are constants

②增加代码可读性 

③增加类型检查,更加严谨

二、联合（共用体）

1、定义

2、Features and Benefits

①共用同一块空间

②union type size

---

一、枚举

1、定义

枚举,is to list all the possibilities.

such as a week7天,We can customize an enumeration type Day,To Monday to Sunday enumerated type.

 2、Features and Benefits

①Values ​​in enumeration types are constants

 

②增加代码可读性 

The following is a simple address book code,The program needs to perform different functions according to the user's choice.

#include <stdio.h>

void menu()
{
	printf("-----------------------------");
	printf("-----1.add      2.delet -----");
	printf("-----3.search   4.modify-----");
	printf("-----5.show     6.empty -----");
	printf("-----7.sort     0.exit  -----");
	printf("-----------------------------");
}

int main()
{
	int input = 0;
	do
	{
		menu();
		printf("请选择：>");
		switch (input)
		{
		case 1:
			Add();
			break;
		case 2:
			Delet();
			break;
		case 3:
			Search();
			break;
		case 4:
			Modify();
			break;
		case 5:
			Show();
			break;
		case 6:
			Empty();
			break;
		case 7:
			Sort();
			break;
		case 0:
			printf("程序退出！\n");
			break;
		default:
			printf("输入错误！\n");
			break;
		}

	} while (input);
	return 0;
}

 可以发现在switch,caseStatements in the code a little abstract,For example when we writecase 7: 的时候,We may have forgotten7代表什么,need to flip up,而这个代码,可以优化.

#include <stdio.h>

void menu()
{
	printf("-----------------------------");
	printf("-----1.add      2.delet -----");
	printf("-----3.search   4.modify-----");
	printf("-----5.show     6.empty -----");
	printf("-----7.sort     0.exit  -----");
	printf("-----------------------------");
}

enum MENU
{
	EXIT,
	ADD,
	DELET,
	SEARCH,
	MODIFY,
	SHOW,
	EMPTY,
	SORT
};
int main()
{
	int input = 0;
	do
	{
		menu();
		printf("请选择：>");
		switch (input)
		{
		case ADD:
			Add();
			break;
		case DELET:
			Delet();
			break;
		case SEARCH:
			Search();
			break;
		case MODIFY:
			Modify();
			break;
		case SHOW:
			Show();
			break;
		case EMPTY:
			Empty();
			break;
		case SORT:
			Sort();
			break;
		case EXIT:
			printf("程序退出！\n");
			break;
		default:
			printf("输入错误！\n");
			break;
		}

	} while (input);

	return 0;
}

这样在case时,Very clear know what to do next.

③增加类型检查,更加严谨

This feature is mainly reflected inC++中,如下：

在C++语言中,Enumeration types can only be assigned their own defined values,and cannot assign other types of values,更严谨 .

二、联合（共用体）

1、定义

Variables defined by union types are the same as structs,Can be of many different types,区别在于,Member variables of union types share the same space,So share body joint is also called.Below is an example of a complex：

2、Features and Benefits

①共用同一块空间

 

可以看到,它们的地址都是一样的,同样,If you change the value of one of the variables,Another variable may also change.

So union type variables are used when,Usually only one of the values ​​is used,or in some special cases.

②union type size

The article has already mentioned,Member variables of union types share the same space,So is the size of the union type the size of the largest member?？请看下面一段代码：

#include <stdio.h>
typedef union Un
{
	char arr[5];
	int i;
}Un;

int main()
{
	printf("%d", sizeof(Un));
	return 0;
}

是不是char arr[5]占5个字节,int i 占4个字节,If it is bigger, it is5呢？

但运行结果却是 8,这是因为,Unions are the same as structs,memory alignment,char arr[5] 的对齐数是1,int i 的对齐数是4,所以UnThe size of the footprint is must4的倍数,也就是8个字节.