️C The structural types of language

Only creation is happiness , Only creatures created are creatures ----- Roman · Roland

Why is there a construction type

stay C In the language we have learned int、float、char Basic data type , These data types can only It means that there are only some specific data , It can't completely represent everything in reality . For example, you want to express a person's characteristics , Then the name of this person , Age , Gender and other information cannot be completed with basic data types alone .

therefore C Language provides constructive types for us to create , Thus, it can express the characteristics of all things in the world .

Next , We will go into detail about Structure （struct）、 enumeration （enum）、 Consortium （union）

Structure

1、 What is a structure

A structure is made up of a group called members （ Member variables ） Composed of different data , Each of these members can have a different type . Structures are usually used to represent several data of different types but related --------- Baidu Encyclopedia

The members of the structure can be Array 、 Pointer to the variable 、 Even other structures , Structures are generally used to describe complex objects .

2、 Writing and format of structure

The structure adopts keyword struct To build

struct Peo   //struct Keyword for structure , Used to create structures  Peo Is the structure name 
{
    
  char name[20];     // In curly brackets is the list of structure members , Also called member variable 
  int age;
  char id[20];
}xiaoming;     //xiaoming To use a structure Peo Structure variables created , But this is a global variable , Not recommended . Note that the semicolon here cannot be lost ！！！

3、 Structure variable definition and initialization

The definition of structure variables can be directly created by using the constructed structure .

The initialization of structure variables adopts { }, Between member variables commas Can then .

The specific code is as follows ：

struct Stu
{
    
    char name[20];
    int age;
    char id[20];
}p1, p2;  //p1,p2 To express with struct Stu Two global variables created , Try to avoid global variables when writing code 
struct Stu p3;
struct Stu p4;  //p3,p4 For use struct Stu Two global variables created 
int main()
{
    
   struct Stu s1;   // Use struct Stu Create local variables 
   struct Stu s2 = {
    "zhangsan", 20, "1213141"}; // Use struct Stu Create local variables and initialize 
    return 0;
}

4、 Access to structure members

①： use == Dot operator （.）== To access structure members

（.） Receive two operands

struct Stu
{
    
    char name[20];
    int age;
    char id[20];
};
int main()
{
    
   struct Stu s2 = {
    "zhangsan", 20, "1213141"}; 
   printf("%s %d %s",s2.name, s2.age, s2.id);  // Use (.) Operator to access structure members 
   return 0;
}

②： use ( -> ) The operator To access structure members

When what we get is not a structural variable , It is a pointer to a structural variable , And then we can use it （ -> ） Operator to access structure members

struct Stu
{
    
    char name[20];
    int age;
    char id[20];
};
void print(struct Stu* s2)   // Parameters are received with structure pointers 
{
    
    printf("%s %d %s\n",s2->name,s2->age,s2->id );  // Pointer of structure variable is ( -> ) Operator to access structure members 
}
int main()
{
    
   struct Stu s2 = {
    "zhangsan", 20, "1213141"}; 
   print(&s2);  // Pass the address of the structure variable to print function 
   return 0;
}

5、 Structure memory alignment （ Calculate the size of the structure ）

Since the structure type is also a data type , Of course, you can also pass sizeof To calculate the size of the structure

But unlike the basic data type , The size of the structure is not constant , It has a great relationship with the type of structure member variables .

Calculation rules ：

①： The first member is associated with the structure variable The offset for the 0 The address of

②： Other member variables should be offset to Align numbers An integral multiple of the address of .

​ Align numbers = The compiler defaults to an alignment number and the smaller value of the member size （vs The default number of alignments is 8, Other compilers generally do not have a default alignment number ）

③： The total size of the structure is Maximum number of alignments （ Each member 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 overall size of the structure is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple

give an example ：

struct S1
{
    
	char c1;
	int i;
	char c2;
};

6、 matters needing attention

①： Because the structure has memory alignment problems , So when designing the structure , Try to gather the types with small space

②： The default alignment number can be through #pragma pack（number） Make changes .

③： When the structure passes parameters , It should be transmitted to the structure Address , It's not worth it , That is, address transmission without value transmission .

enumeration

1、 Enumeration writing and format

Enumeration adopts keyword enum To build

enum Day   // enum For enumerating keywords , Used to create enumeration types ,Day Enumeration name 
{
    
	mon,  // Those in curly brackets are possible values , The alignment is listed one by one .
	Tues,  // Separate them with commas 
	Wed,
	Thur,
	Fri,
	Sat,
	Sun
};

In enumeration , All possible value types in curly braces have values , The default first one is 0, Increase in order , You can also align for initial assignment .

union （ Shared body ）

1、 What is union

Federation is also a special custom type , Variables defined by this type also contain a series of members , The characteristic is that these members Share a memory space ( So union is also called community )

2、 Joint writing and format

Joint adoption union Keyword to create

union Un  //  The basic writing format is the same as the structure 
{
    
 char c;
 int i;
};  

3、 Calculation and characteristics of joint size

The size of the union is different from the basic data type , Even different from the calculation rules of the structure .

Calculation rules of joint size ：

①： The size of the union is at least the size of the largest member

②： When the maximum member size is not an integral multiple of the maximum number of alignments , It's about aligning to an integer multiple of the maximum number of alignments

give an example ：

union Un1
{
    
	char c[5];
	int i;
};
union Un2
{
    
	short c[7];
	int i;
};
int main()
{
    
	printf("%d\n", sizeof(union Un1));   // The size is 8
	printf("%d\n", sizeof(union Un2));  // The size is 16
	return 0;
}

Characteristics of Union ：

①： When the value of a member variable in the union changes , The values of other member variables will also change .

②： The size of the union is at least the size of the largest member variable .

summary

Custom types give us the ability to represent a variety of things , So that we have the ability to innovate . Each type has its value and significance , We should find and excavate the proper use of these types , Let's take our code capability to the next level ！

come on. ！