C language: summary of consortium knowledge points

The joint declaration is similar to the structure declaration , however , Members of the Federation share the same storage space , And there can only be one member in the union at a time .

• Joint profile

  union （union） It's a data type , It can store different data types in the same memory space （ Not stored at the same time ）. Some people call him the common body , It means sharing the same storage space .

• Create a Federation

  union hold{
        
      int dight;
      double bigfl;
      char letter;
  }fit;
  

  The structure declared according to the above form can store a int Type data ,double Type data ,char Type data . But the declared union can only store one int Value of type or a double The value of type or a char Type value .

• Use Union

  Here are some uses of Union ：

  fit.dight = 23;// hold 23 Stored in fit, Occupy 2 Bytes 
  fit.bigfl = 2.0;// eliminate 23, Storage 2.0, Occupy 8 Bytes 
  fit.letter = 'h';// eliminate 2.0, Storage h, Occupy 1 Bytes 
  

  In the Union , Store one value at a time . Even if there's enough space , You cannot store one at the same time char Value of type and one int Type value . This point needs to be understood , This is the biggest difference between a consortium and a structure . The following code helps you understand .

#inlude <stdio.h>

// Consortium template 
union hold{
    
    int dight;
    double bigfl;
    char letter;
}fit;

int main()
{
    
	fit.bigfl = 2.0;  // eliminate 23, Storage 2.0, Occupy 8 Bytes 
	fit.dight = 23;   // hold 23 Stored in fit, Occupy 2 Bytes 
	fit.letter = 'h'; // eliminate 2.0, Storage h, Occupy 1 Bytes 

	printf("union demo:%f\n",fit.bigfl); 	
	printf("union demo:%d\n",fit.dight); 
	printf("union demo:%c\n",fit.letter); 
}

If you run this code, you will find strange phenomena , Although each member is assigned a value , But the print did not print out , This is the characteristic of the consortium , Only one type can be stored at a time , When the next type is stored, the previous type is eliminated .（ Even if it is not eliminated, it is not recommended to store two data types at the same time ）.

Then the consortium feels useless , Only one data type can be stored at a time , It's better to use structure . ha-ha , Existence is reason . In some scenarios, the consortium is still very useful . For example, the following scene ：

Please complete this requirement , It is required that teachers' information can be stored （ full name , Age , Gender , Subjects taught ）, And students' information （ full name , Age , Gender , class ）. You must have a knowing smile , I thought it was not difficult for you , Just pick up the pen .

// Teacher's template 
struct Teaher{
    
	char name[20];
	int old;
	char sex;
	char object[20];
}TEACHER;

// Student templates 
struct Student{
    
	char name[20];
	int old;
	char sex;
	char class[20];
}STUDENT;

Be accomplished , But ... Is this a bit of a waste of space ？ Their structure is just different in the final profession , Everything else is the same , If there are many similar templates , Do we each need to create one ？ Obviously not . At this time, the community appeared , Let's see .

struct comment{
    
	char name[20];
	int old;
	char sex;
	union {
    
		char object[20];
		char class[20];
	};
}T;

This perfectly solves this need , By the way , Such a structure becomes anonymous Association （ That is, structure members without names ）. If you want to access the members of the community , have access to T.object perhaps T.class To assign values to them .