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\n",order.amount.volume,order.name);else if (order.units == POUNDS)printf("%2.2f lbs of %s\n",order.amount.weight,order.name);elseprintf("%i %s\n",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; }