Difference and connection between structure and Consortium

Define a structure as follows B And a consortium U ：

// Defining structure 
typedef struct{
    
    int i;
    double j;
}B;

// Define a consortium 
typedef union{
    
    int i;
    double j;
}U;

Can pass sizeof() Function to check the memory size of structures and federations .sizeof(B) The value of is 12,sizeof(U) The value of is 8 instead of 12 . Why? sizeof(U) No 12 Well ？ because union All members in share memory ,i and j The memory of is the same . And the overall memory size is divided by the members of the maximum memory , therefore U The memory size of is double Size .

therefore , We can also see that all members of the consortium share memory , And only one member can get the right to use this memory at the same time （ That is, read and write to memory ）, Each member of the structure has its own memory , Use each other without interference .

Understand the concept of joint memory , You can see why you can only assign values to one member at a time , Because if you assign another value , Will overwrite the value of the previous member . All in all , The difference between the two can be summed up in the following two points .

• （1） Both structures and unions are made up of members of different data types , But at any moment , Only one selected member can be stored in the union , And all the members of the structure exist .
• （2） Assign values to different members of the consortium , Will be rewritten for other members , The value of the original member does not exist , The assignment to different members of a structure does not affect each other .