User defined data type - structure

Custom data types

Structure

enumeration

Consortium

char,short,int long,float,double Built in type ——c The language has its own data types .
Structure declaration ：

struct tag
{
    
member-list  Member list 
};variable-list  Variable list 

Declare a student type , Create student variables by student type （ object ）

struct stu
{
    
  char name[20];
  char tale[12];
  char sex[10];
  int age;
  }s4,s5,s6;// You can't lose the semicolon --- Global variables 
  struct stu s3;//--- Global variables 
int main()
{
    
  struct stu s1;//--- local variable 
  struct stu s2;//--- local variable 
  return 0;
}

Anonymous struct type ：

struct
{
    
  int a;
  char b;
}x;//x Can't save 

The compiler treats two struct declarations as two completely different types , therefore p=&x It's illegal.

data structure ： The storage structure of data in memory –《 Linked list 》

typedef struct Node//Node Self reference cannot be omitted 
{
    
  int data;
  struct Node *next;// Find variables of the same type 
  }Node;
  int main()
  {
    
  struct Node n1;
  Node n2;
  return 0;
  }

Structure alignment rules ：

1. The first member is offset from the structure variable by 0 The address of .
2. Other member variables should be aligned to a certain number （ Align numbers ） An integral multiple of the address of .
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 , Align nested structures to their maximum number of alignments （ The number of alignments with nested structures ） Integer multiple .

visual The default value in is 8
Align numbers = The compiler defaults to an alignment number and the smaller value of the member size

#pragma pack(4)// Set the default alignment digit 4
struct S
{
    
  char c1;//1 Bytes 
  double d;//8 Bytes 
 };
 #pragma park()// Unset the default number of alignments 

C Language offsetof function ： The return value of this macro is of type size-t, The offset value of this type member , Such as （printf"%d\n",offsetof(struct s,i）);

The structure parameters shall be addressed as much as possible ：
When a function passes parameters , The parameter is stack pressing , There will be time and space overhead . If you pass a structure object , The structure is too large , The system overhead of parameter stack pressing is relatively large , So it leads to performance degradation .

struct S
{
    
  int a;
  char c;
  double d;
  };
  void Init(struct S tmp)
   {
    
      tmp.a=100;
      tmp.c='w';
      tmp.d=3.14;
    }
    void Print1(struct Stmp)
    {
    
    printf("%d %s %lf\n",tmp.a,tmp.c,tmp.d);
    }
      void Print 2(const struct S* ps)// Can be in ps Before to add const, Still address , But it cannot be changed S Value 
      {
    
          printf("%d %c %lf\n",ps->a,ps->c,ps->d);
     }
     int main()
    {
    
       struct S s={
    0};
       Init (&s);// Must be address passing , Otherwise tmp Change will not affect s Change in value 
       Print 1(s);// Memory doesn't matter 
       Print 2(&s);
       return 0;
    }