Union is also like structure, i.e. collection of different data types which are grouped together. Each element in a structure or a union is called member.

•    Structure allocates storage space for all its members separately.

•    Whereas, Union allocates one common storage space for all its members

无论是基本数据类型还是数组都仅仅描述了事物某一方面的特性, 但是, 一种事物往往具有多方面的属性, 如一个同学有学号, 姓名, 性别, 年龄等属性.

定义形式:

struct union_type_name

{

    member_type        member_name_1;

    member_type        member_name_2;

    ...

    member_type        member_name_n;

};

结构体变量的定义 & 结构体成员的初始化, 例如:

//结构体变量名.成员名

#include <iostream>

using namespace std;

void main()

{

    struct person_info

    {

        int index;

        char name[30];

        short age;

    };

    person_info p_info;        //也可以写成struct person_info p_info;

    p_info.index=1;

    strcpy(p_info.name,"Jim");    //位字符数组赋值需要使用字符串复制函数strcpy(), 否则会提示: cannot convert from 'char [4]' to 'char [30]'

    p_info.age=20;

    cout << p_info.index<< endl;

    cout << p_info.name << endl;

    cout << p_info.age << endl;

    cout<<sizeof(person_info)<<endl;

    cout<<sizeof(p_info)<<endl;        //结构体类型和结构体变量的大小都是36字节, 即4+30+2=36

}

/*

可以在声明一个结构体变量后赋值:

struct person_info

{

    int index;

    char name[30];

    short age;

};

person_info p_info={1,"Jim", 20};

*/

/* 也可以在定义结构体变量时直接对成员赋值:

struct person_info

{

    int index;

    char name[30];

    short age;

} p_info={1,"Jim", 20};*/

//结构体指针变量->成员名

//在定义结构体的同时, 声明结构体指针变量

#include <iostream>

using namespace std;

void main()

{

    struct person_struct

    {

        int index;

        char name[30];

        short age;

    }*p, p_info={1,"Jim",20};

    p=&p_info;

    cout <<p->index << endl;

    cout <<p->name << endl;

    cout << p->age << endl;

    cout <<(*p).index << endl;

    cout <<(*p).name << endl;

    cout <<(*p).age << endl;

    cout << sizeof(p) << endl;

    cout << sizeof(p_info) << endl;

}

//也可以写成:

struct person_info

{

    int index;

    char name[30];

    short age;

};

person_info p_info={1,"Jim",20};

struct person_info *p=&p_info; //一定要写定义的结构体类型名, 不能写成struct *p=&p_info;

总之, 在上述案例中, 访问结构体成员有下面三种方法:

① p_info.index; ② p->index; ③ (*p).index;

② 当一个结构体类型定义完成后, 如果要使用这个类型, 保留字struct在C语言中必须使用，而在C++中则可被省略不写。

#include <iostream>

using namespace std;

void main()

{

    struct person_info

    {

        int index;

        char name[30];

        short age;

        struct work_place

        {

            char address[150];

            char postCode[30];

            char gateCode[50];

            char street[100];

            char area[50];

        }WP; //子结构体(sub-structure)

};

    person_info p_info;    //p_info是person_info结构体类型变量

    strcpy(p_info.WP.address,"House");    //可以把WP看作是p_info变量的子变量(sub-variable)

    strcpy(p_info.WP.postCode,"10000");

    strcpy(p_info.WP.gateCode,"302");

    strcpy(p_info.WP.street,"Lan Tian");

    strcpy(p_info.WP.area,"China");

    cout << p_info.WP.address << endl;

    cout << p_info.WP.postCode << endl;

    cout << p_info.WP.gateCode<< endl;

    cout << p_info.WP.street << endl;

    cout << p_info.WP.area << endl;

}

#include <iostream>

using namespace std;

struct person_info //定义结构体

{

    int index;

    char name[30];

    short age;

};

void show_stuct_message(struct person_info my_info) //自定义函数，形参是结构体变量my_info, 输出结构体变量成员, 不返回值

{

    cout << my_info.index << endl;

    cout << my_info.name << endl;

    cout << my_info.age<< endl;

}

void main()

{

    person_info p_info; //声明结构体变量p_info, 作为形参

    p_info.index=1;

    strcpy(p_info.name,"Jim");

    p_info.age=20;

    show_stuct_message(p_info); //调用自定义函数

}

#include <iostream>

using namespace std;

struct person_info

{

    int index;

    char name[30];

    short age;

};

void show_struct_message(struct person_info *my_info)

{

    cout << my_info->index << endl;

    cout << my_info->name << endl;

    cout << my_info->age<< endl;

}

void main()

{

    person_info p_info;

    p_info.index=1;

    strcpy(p_info.name,"Jim");

    p_info.age=20;

    show_struct_message(&p_info);

}

结构体数组: 每个元素都是结构体变量的数组

//定义结构体数组的一般形式:

struct 结构体名

{

    成员列表;

}数组名;

struct Student    //可以把Student这个结构体名省略

{

    char name[20];

    int number;

    char sex;

    int grade;

}student[5];

//结构体数组中各数据在内存中的存储是连续的,例如:

#include <iostream>

using namespace std;

void main()

{

    struct person_info

    {

        int index;

        char name[30];

        short age;

    }p_info[5]={{1,"Jim",20},    //p_ifno是一个结构体数组

                {2,"Eric",21},

                {3,"Peter",22},

                {4,"Amy",22},

                {5,"Lucy",22}};

    struct person_info *p;

    p=p_info;    //p_info是一个结构体数组名, 因此就代表一个地址值

    for(int i=0;i<5;i++,p++)

    {

        cout << p->index << endl;

        cout << p->name << endl;

        cout << p->age << endl;

    }

}

所有共用体在同一时刻只能有一个值, 它属于某一数据成员, 不过其它成员会共享这个值, 只是会转换成这些成员的数据类型

共用体变量的大小是其最大成员的大小.

联合体使用场景:

例如通信中的数据包会用到共用体, 因为不知道对方会发一个什么数据类型的包过来，用共用体的话就很简单了，定义几种格式的包，收到包之后就可以直接根据包的数据类型取出数据。

定义形式:

union union_type_name

{

    member_type        member_name_1;

    member_type        member_name_2;

    ...

    member_type        member_name_n;

};

联合体变量的定义, 例如:

#include<stdio.h>

union data_union            /*声明共用体类型*/

{

    int a;            /*成员变量*/

    char b;            

};

int main()

{

    union data_union my_union;        /*定义共用体变量*/

    my_union.a=97;                /*为共用体变量中成员赋值*/

    printf("a: %d\n",my_union.a);        /*输出成员变量数据*/

    printf("b: %c\n",my_union.b);

    my_union.b='A';            /*改变成员的数据*/

    printf("a: %d\n",my_union.a);        /*输出成员变量数据*/

    printf("b: %c\n",my_union.b);

    printf("sizeof(data_union): %d\n",<