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# 6.5 联合 Unions
### 6.5.1 联合的基本概念
联合 Unions 允许你在同一内存位置存储不同的数据类型。和结构体（struct）不同的是，**联合体中的成员共享同一块内存空间**。因为，编译器最只为联合中最大的成员分配足够的内存空间。 **注意使用联合体的风险：**
* 数据共享风险： 联合体的不同成员共享同一块内存，容易导致数据混淆或覆盖，因此使用时需要格外小心。
* 只能同时存储一个值： 一次只能存储一个成员的值，存入新值会覆盖原来的值。
* 对齐和大小： 联合体的大小和对齐方式取决于最大的成员。
### 6.5.2 用联合来节省空间的例子
{% code title="6.5.1 union-with-struct-fucked.c" lineNumbers="true" %}
```c
#include
#include
// 定义一个结构体
struct Employee {
int id;
char name[20];
char department[30];
union {
float salary;
struct {
int hours_worked;
int rate_per_hour;
} hourly_payment;
} payment_info;
};
int main() {
// 定义一个 Employee 结构体变量
struct Employee emp1;
// 设置员工信息
emp1.id = 1001;
strcpy(emp1.name, "John Doe");
strcpy(emp1.department, "Engineering");
// 设置薪资信息（这里使用联合来节省空间）
emp1.payment_info.salary = 5000.0; // 设置月薪
// 打印员工信息
printf("Employee ID: %d\n", emp1.id);
printf("Name: %s\n", emp1.name);
printf("Department: %s\n", emp1.department);
// 利用联合来访问薪资信息
printf("Salary: $%.2f\n", emp1.payment_info.salary);
// 如果员工是按时薪支付
emp1.payment_info.hourly_payment.hours_worked = 40;
emp1.payment_info.hourly_payment.rate_per_hour = 20;
printf("Hours Worked: %d\n", emp1.payment_info.hourly_payment.hours_worked);
printf("Rate per Hour: $%d\n", emp1.payment_info.hourly_payment.rate_per_hour);
return 0;
}
```
{% endcode %}
```c
Employee ID: 1001
Name: John Doe
Department: Engineering
Salary: $5000.00
Hours Worked: 40
Rate per Hour: $20
```
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