Detailed explanation of structure

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• 1. Declaration of a structure

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• struct tag
  {
   member-list; 
  }variable-list;

  tag It can be understood as the label of structure ,variable-list Is the name of the structure , What is declared in braces is the data type , as follows

• struct Stu
  {
   char name[20];// name 
   int age;// Age 
   char sex[5];// Gender 
   char id[20];// Student number 
  }; // You can't lose the semicolon 

  There are also special statements , Anonymous struct type ：

  // Anonymous struct type 
  struct
  {
   int a;
   char b;
   float c; }x;

• 2. Self reference of structure

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• 

  It can be understood as a structure containing data fields and pointer fields , But the pointer field may be empty , ad locum ,struct Node*next It is a pointing type Node Structure pointer of , So it is the self reference of the structure .

• 3. Rename the structure  

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• 

  utilize typedef Name the structure weight , This can save a lot of redundant code . 

• 4. Structure variable definition and initialization （ Examples ）

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• struct Point
  {
   int x;
   int y;
   }p1; // Define variables while declaring types p1
  struct Point p2; // Define structure variables p2
  // initialization ： Define variables and assign initial values at the same time .
  struct Point p3 = {x, y};
  struct Stu        // Type declaration 
  {
   char name[15];// name 
   int age;      // Age 
  };
  struct Stu s = {"zhangsan", 20};// initialization 
  struct Node
  {
   int data;
   struct Point p;
   struct Node* next; 
  }n1 = {10, {4,5}, NULL}; // Structure nesting initialization 
  struct Node n2 = {20, {5, 6}, NULL};// Structure nesting initialization 

  5. Memory alignment of structures （ Calculate the size of the structure ）

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    For its rules ：1. The first member is offset from the structure variable by 0 The address of .

     2. Other member variables are aligned to a number （ Align numbers ） An integral multiple of the address of .

      Align numbers = Compiler default alignment number And The size of the member Smaller value .

     VS The default value in is 8

     3. The total size of the structure is the maximum number of alignments （ Each member variable has an alignment number ） Integer multiple .

     4. If the structure is nested , The nested structure is aligned to an integral multiple of its maximum alignment , The integrity of the structure

      Body size is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple .

// practice 1
struct S1
{
 char c1;
 int i;
 char c2;
};
printf("%d\n", sizeof(struct S1));// The result is 12

struct S3
{
 double d;
 char c;
 int i;
};
printf("%d\n", sizeof(struct S3));

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6. Change the default alignment number

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#include <stdio.h>
#pragma pack(8)// Set the default alignment number to 8
struct S1
{
 char c1;
 int i;
 char c2;
};
#pragma pack()// Unset the default number of alignments , Restore to default 
#pragma pack(1)// Set the default alignment number to 1