【C Advanced】11, enum, typedef, sizeof

Summary

is a 1616049c81b878 custom type in the C language; variables of the enum type can only take the discrete value when defined.

can define 1616049c81b896 real constants in C language. Enumerations are commonly used in engineering to define constants (unnamed enumerations).

3) sizeof is the built-in indicator compiler, and the value of sizeof is determined at compile time.

4) sizeof is a built-in keyword of C language and is not a function

• During the compilation process, all sizeof will be replaced by specific values. The functions or statements in parentheses will not be executed.
• program has nothing to do with sizeof

5) Typedef is used to existing types . In essence, 1616049c81b985 cannot generate new types. The new type name cannot be modified by unsigned or signed. The can be defined after typedef.

enum, typedef, siezof analysis

1、enum

• is a 1616049c81b9da custom type in C language
• The value of enum is an integer value that can be customized according to needs
• first defined enum value defaults to 0
• The default enum value is added 1 to the previous defined value of

• Variables of enum type can only take discrete values at the time of definition

  enum Color
  {
    RED ,
    YELLOW = 2,
    GREEN
  };
  
  int main()
  {
    enum Color cc = GREEN;
  
    printf("cc = %d\n", cc);   // 3
  
    return 0;
  }

• The value defined in enum is the real constant

• In engineering, enum is mostly used to define plastic constants

  enum                // 无名枚举
  { 
    ARRAY_SIZE = 5    // 用来定义数组大小，常量
  };
  
  int arr[ARRAY_SIZE] = {0};
  

2、sizeof

• sizeof is a built-in indicator of the
• The value of sizeof has been determined at compile time

• sizeof is used to calculate the memory size occupied by type or variable

  • sizeof is used for type: sizeof(type)

  • sizeof is used for variables: sizeof(var) or sizeof var

    int var = 0;
    printf("%d\n", sizeof(int));
    printf("%d\n", sizeof(var));
    printf("%d\n", sizeof var);

• sizeof is a built-in keyword of C language and is not a function

  • All sizeof will be replaced by specific values during compilation

  • program has nothing to do with sizeof

    // 下面程序的输出是？
    int var = 0;
    int size = sizeof(var++);
    printf("var = %d, size = %d\n", var, size);        // 0, 4

  The program output var = 0, size = 4, because sizeof has been determined at compile time, , that is, sizeof(var++) is replaced with 4, and var++ will not be executed again.

another example:

int func()
{
    printf("sizeof will be replaced in compile period!!");
    return 0;
}

int main()
{
    int size = sizeof(func());
    printf("size = %d\n", size);   // 4
    return 0;
}

Output: size = 4, but func(), indicating that during compile time, sizeof(func()) is replaced with 4, and func() is not called.

3、typedef

• typedef is used to existing data type 1616049c81be1b
• typedef essentially cannot generate new types

• Types renamed by typedef:

  • Can be defined after the typedef statement, original type can be defined after the typedef statement

  • Cannot be modified by unsigned or signed, cannot be used with unsigned or signed when the new name is used

    typedef int Int32;
    
    typedef struct _tag_point Point;    // _tag_point在后面才定义
                                  // 此处只是告诉编译器，在使用的地方看到Point，知道他就是_tag_point
    struct _tag_point
    {
     int x;
     int y;
    };
    
    typedef struct 
    {
     int len;
     int array[];
    } SoftArray;
    
    
    int main()
    {
     Int32 i = 100;
    //    unsigned Int32 ii = 0;  // error，不能用unsigned修饰
    
     SoftArray* sa = NULL;
     sa = (SoftArray*)malloc(sizeof(SoftArray) + 5*sizeof(int));
     sa->len = 5;
    
     Point pp;
    
     return 0;
    }

This article is summarized from "C Language Advanced Course" by Tang Zuolin from "Ditai Software Academy".
If there are any errors or omissions, please correct me.