联合体union 枚举enum

联合体 union

• 不同类型的变量共同占用一段内存(相互覆盖),联合变量任何时刻只有一个成员存在，节省内存
• 联合体变量的大小＝最大的成员所占的字节数

#include "stdlib.h"
#include "stdio.h"
union Location{
int x;
int y;
    double z;
}
void main(){
    union Location l1;
    l1.x = 100;
    l1.y = 80;
    l1.z = 18.9;

printf("%d, %d, %f\n", l1.x, l1.y, l1.z);
system("pause");
}

typedef union jvalue{
    jboolean z;
    jbyte b;
    jcahr c;
    jshort s;
    jint i;
    jlong l;
    jfloat f;
    jdouble d;
    jobject l;
} jvalue;

枚举 enum

限定值，保证取值的安全性

//enumeration
enum Day{
    Monday = 0,
    Tuesday = 1,
    Wednesday = 2,
    Thursday = 3,
    Friday = 4,
    Saturday = 5,
    Sunday = 6
}

enum Day{
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

void main(){
//枚举类的值，必须是括号中的值
enum Day d = Monday;
printf("%#x, %d\n", &d, d);
    getchar();
}