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目录
一、结构体
1.结构体struct
2.?结构体成员对齐
二、联合体
三、枚举
四、综合运用（L1范数为例）
五、typedef
一、结构体
1.结构体struct
把几个不同类型的成员打包在一起，形成新的数据类型
struct Student{
char name[4];
int born;
bool male;
};
struct Student stu;//C语言
Student stu;//C++
strcpy(stu.name, "Yu");用.操作成员变量
stu.born = 2000;
stu.male = true;
struct Student stu = {"Yu", 2000, true};//初始化
struct Student students[100];//结构体数组，里面有100个结构体
students[50].born = 2002;
| | | | | 13 | | | 12 | | | 11 | | | 10 | | | 9 | | male | 1 | 8 | | born | 2000 | 7 | | 6 | | 5 | | 4 | | name | 0 | 3 | | 0 | 2 | | 'u' | 1 | | 'Y' | 0 | | | -1 | | | -2 | | | |
2.?结构体成员对齐
struct Student1{
int id;
bool male;
char label;
float height;
};
| | | | | | | height | | 11 | | 10 | | 9 | | 8 | | | 7 | | | 6 | | label | | 5 | | male | | 4 | | id | | 3 | | 2 | | 1 | | 0 | | | | | | | | | | |
为了io方便把变量对齐，绝对0地址，都是4的倍数，8的倍数height到6，一次性就装不完，装两次，后面读写效率更高?
struct Student2{
int id;
bool male;
float height;
char label;
};
| | | | | | | | 15 | | | 14 | | | 13 | | label | | 12 | | weight | | 11 | | 10 | | 9 | | 8 | | | 7 | | | 6 | | | 5 | | male | | 4 | | id | | 3 | | 2 | | 1 | | 0 | | | | | | |
一般4个字节对齐
二、联合体
一个IP地址
union ipv4address{
std::uint32_t address32;
std::uint8_t address8[4];
};
联合体里面所有成员共享同一个内存
两个变量首地址相同，以最大的成员变量为准
sizeof(union ipv4address) is 4.
| | Union | Address | | | | | | | | p+4 | | address8[3] | address32 | 127 | p+3 | | address8[2] | 0 | p+2 | | address8[1] | 0 | p+1 | | address8[0] | 1 | p+0 | | | | p-1 | | | | |
三、枚举
替换const
enum color {WHITE, BLACK, RED, GREEN, BLUE, YELLOW, NUM_COLORS};
enum color pen_color = RED;
pen_color = color(3);
cout << "We have " << NUM_COLORS << " pens." << endl;
pen_color += 1; //error!枚举类型不可做算数运算
int color_index = pen_color;
color_index += 1;
cout << "color_index = " << color_index << endl;
四、综合运用（L1范数为例）
L1范数
enum datatype {TYPE_INT8=1, TYPE_INT16=2, TYPE_INT32=4, TYPE_INT64=8};
//point用来表示3维世界的向量（x,y,z），向量用union表示，向量的数据类型可能不同，该用那个由枚举类型决定
struct Point{
enum datatype type;
union {
std::int8_t data8[3];
std::int16_t data16[3];
std::int32_t data32[3];
std::int64_t data64[3];
};
};
size_t datawidth(struct Point pt)
{
return size_t(pt.type) * 3;
}
int64_t l1norm(struct Point pt)//L1范数
{
int64_t result = 0;
switch(pt.type)
{
case (TYPE_INT8):
result = abs(pt.data8[0]) +
abs(pt.data8[1]) +
abs(pt.data8[2]);
break;
...
五、typedef
使用 typedef 为现有类型创建别名
typedef int myint;
typedef unsigned char vec3b[3];
typedef struct _rgb_struct{//name _rgb_struct can be omit
unsigned char r;
unsigned char g;
unsigned char b;
} rgb_struct;
myint num = 32;
unsigned char color[3];
vec3b color = {255, 0, 255}; //color是个数组，和上面一样
rgb_struct rgb = {0, 255, 128};
常用在系统头文件
_uint8_t.h
#ifndef _UINT8_T
#define _UINT8_T
typedef unsigned char uint8_t;
#endif /* _UINT8_T */
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