C Language Prerequisites: typedef and typedef struct

typedef

typedef existing new;

The function of typedef is to create a new data type name, for example:

typedef int Length;  // 数据类型

typedef char *String;  // 字符指针

typedef struct tnode *Treeptr;  // 指针

typedef int (*PFI) (char *, char *); // 指向函数的指针

Length l = 120;
printf("%d \n", l);

A typedef declaration does not create a new type; it merely adds a new name for an existing type.

Two key benefits of using typedef are:

• It can parameterize the program, improving the program's portability.
• It can provide better descriptiveness for the program, increasing readability.

struct

struct is used to define a new data type that contains multiple members. There are the following 4 ways to define it:

Method 1: Do not specify a type name, declare the struct variable directly (not recommended)

struct {
    int age;
} student;

student.age = 18;
printf("%d \n", student.age);

Method 2: First define the type name, then declare the struct variable

Characteristic: You need to use the struct type name every time to declare a variable.

struct Student {
    char name;
    int age;
};

struct Student student1;
student1.age = 20;

Method 3: Define the type name and declare the struct variable

struct HeadTeacher {
    int age;
} h1;

h1.age = 50;
struct HeadTeacher h2;
h2.age = 70;
printf("head teacher1 age = %d \n", h1.age);
printf("head teacher2 age = %d \n", h2.age);

Method 4: Define the type name and give it an alias (recommended)

typedef struct {
    int age;
} Teacher;

Teacher t;
t.age = 30;
printf("teacher age = %d \n", t.age);