联合体

1.联合：计算机为其中最大的字段分配空间。

例如：

typedef union
{
　　short count;
　　float weight;
　　float volume;
}quantity;     //占4字节

2.设置联合的值

C89方式（保存第一个字段的值）：quantity q = {4};  注意加上{}

指定初始化器（按名设置联合字段的值）：quantity q = {.weight=1.5}; 也能设置结构字段的初值

点表示法：　　quantity q;

　　　　　　　q.volume = 3.7;

3.结构体常和联合体一起使用

 例如：

typedef union
{
    float lemon;
    int lime_pieces;
}lemon_lime;

typedef struct
{
    float tequila;
    float cointreau;
    lemon_lime citrus;
}margarita;

margarita m = {2.0,1.0,{0.5}};
margarita b = {2.0,1.0,{.line_pieces=1}};

4.枚举

enum colors {red,green,puce};

enum colors favorite = puce;

5.结构和联合用分号分割数据项，而枚举用逗号

练习（枚举）

#include <stdio.h>

typedef enum
{
    COUNT,POUNDS,PINTS
}unit_of_measure;

typedef union
{
    short count;
    float weight;
    float volume;
}quantity;

typedef struct
{
    const char *name;
    const char *country;
    quantity amount;
    unit_of_measure units;
}fruit_order;

void display(fruit_order order)
{
    printf("This order contains");
    if(order.units == PINTS)
        printf("%2.2f pints of %s
",order.amount.volume,order.name);
    else if (order.units == POUNDS)
        printf("%2.2f lbs of %s
",order.amount.weight,order.name);
    else
        printf("%i %s
",order.amount.count,order.name);
}

int main()
{
    fruit_order apples = {"apples","England",.amount.count=144,COUNT};
    fruit_order strawberries = {"strawberries","Spain",.amount.weight=17.6,POUNDS};
    fruit_order oj = {"orange juice","U.S.A",.amount.volume=10.5,PINTS};
    display(apples);
    display(strawberries);
    display(oj);
    return 0;
}