One 、 Structure

1.1 Declaration of a structure

Use the example of address book to show ：

struct PeoInfo
{
    
	char name[10];// full name 
	int age;// Age 
	char sex[6];// Gender 
	char tel[13];// Telephone 
	char addr[20];// Address 
}p1;//p1 Global variable 

This is an ordinary statement , There is another way to declare ： Declaration of anonymous structure .

struct
{
    
	char name[10];// full name 
	int age;// Age 
	char sex[6];// Gender 
	char tel[13];// Telephone 
	char addr[20];// Address 
}p1;//p1 Global variable 

Anonymous structures can only create variables after declarations , It cannot be used in other places .

struct
{
    
	int a;
	char b;
	float c;
}x;

struct
{
    
	int a;
	char b;
	float c;
}a[20], * p;

int main()
{
    
	p = &x;
	return 0;
}

Note that this code is illegal , The compiler will regard the above two types as Different types .

1.2 Self reference of structure

struct Node
{
    
	int data;
	struct Node next;
};

Someone might write like this . But that's going to be a problem ： Can't calculate Node Size . So use it correctly ：

struct Node
{
    
	int data;
	struct Node* next;
};

1.3 Initialization of structure variables

struct Node
{
    
	int data;
	struct Node* next;
}n1;
struct Node n2;

Both methods of definition are ok .
Initialization should use {}, Containing nested is {} Riga {}.

struct A
{
    
	int a;
	int b;
};

struct B
{
    
	int c;
	struct A a;
};

int main()
{
    
	struct A a = {
     1, 2 };
	struct B b = {
     1, {
    1, 1} };
	return 0;
}

1.4 Memory alignment and size calculation of structure

There are detailed introductions and exercises ：
Structure / Calculation of the size of the consortium

1.5 Why is there memory alignment ️️

1️⃣ Platform reasons
Not all hardware platforms can access any data on any address ; Some hardware platforms can only access certain types of data at certain addresses , Otherwise, a hardware exception will be thrown .
2️⃣ Performance reasons
data structure ( Especially stacks ) should Align as much as possible on natural boundaries .
The reason lies in , To access unaligned memory , The processor needs to do Two memory accesses ; and Aligned memory access requires only one access .

Suppose that 32 Bit machine （ Access four bytes at a time ） visit a：

On the whole ：
Structure memory alignment is to take Space for time How to do it .

So to save space , We should design ： Let the members who occupy less space gather together as much as possible .

Two 、 Bit segment

2.1 The concept of bit segment

struct A
{
    
	int _a : 2;
	int _b : 5;
	int _c : 10;
	int _d : 30;
};

It can be seen that there are two differences between him and the structure ：

1） The member of the segment must be int、unsigned int or signed int ( It can also be char type ).
2） The member name of the segment is followed by a colon and a number .

2.2 Calculation of bit size

First of all, you need to know what the number after the colon means ：
int Its size is 4 Bytes , The back 2 It only needs two bits . So the following numbers Can't go beyond his own type .
computing method ：

it is to be noted that Unused space is wasted

struct A
{
    
	int a : 1;
	char b;
};

The size of this bit segment is also 8, Because you have to think about Memory alignment And different data types cannot use one space .

2.3 Bit segment memory allocation

struct S
{
    
char a:3;
char b:4;
char c:5;
char d:4;
};
struct S s = {
    0};
s.a = 10;
s.b = 12;
s.c = 3;
s.d = 4;

2.4 The cross platform problem of bit segment

1） int It's uncertain whether a bit segment is treated as a signed number or an unsigned number .
2） The number of the largest bits in the bit segment cannot be determined .（16 The machine is the largest 16,32 The machine is the largest 32, It's written in 27, stay 16 A bit of the machine will go wrong ）.
3） The members in the bit segment are allocated from left to right in memory , Or right to left allocation criteria have not yet been defined .
4） When a structure contains two bit segments , The second segment is relatively large , Cannot hold the remaining bits of the first bit segment , Whether to discard the remaining bits or to use , This is uncertain .

3、 ... and 、 enumeration

3.1 Definition of enumeration type

enum Day// week 
{
    
	Mon,
	Tues,
	Wed,
	Thur,
	Fri,
	Sat,
	Sun
};

Like week , Color, a finite thing, can be enumerated .
enum Day Is enumeration type {} It is called enumeration constant .

3.2 Advantages of enumeration

1） Increase the readability and maintainability of the code
2） and #define The defined identifier comparison enumeration has type checking , More rigorous .
3） Prevent named pollution （ encapsulation ）
4） Easy to debug
5） Easy to use , You can define more than one constant at a time

3.3 Use of enumeration

enum Day// week 
{
    
	Mon,
	Tues,
	Wed,
	Thur,
	Fri,
	Sat,
	Sun
};

In fact, these enumeration constants have values , from 0 Start , Back up +1. You can also assign values by yourself ：

enum Day// week 
{
    
	Mon = 1,
	Tues,
	Wed,
	Thur,
	Fri,
	Sat,
	Sun
};

This is from 1 Back up . The back can be straight Assign values with enumerated variables

Four 、 Consortium ( Shared body )

4.1 Definition of consortium

// Declaration of union type 
union Un
{
    
	char c;
	int i;
};

4.2 Characteristics of the consortium

Because the consortium shares a space , So changing one variable will also change the other .

4.3 Calculation of the size of the consortium

There are detailed introductions and exercises ：
Structure / Calculation of the size of the consortium