Detailed explanation of Union [C language]

We know the structure （Struct） Is a construction type or complex type , It can contain multiple members of different types . stay C In language , There's another syntax that's very similar to the structure , be called Shared body （Union）, It is defined in the form of ：

union Community name {
    Member list
};

Commons are sometimes called unions or consortia , This is also Union The original meaning of this word .

Structure and common body difference lie in ： Each member of the structure takes up different memory , There is no influence on each other ; And all members of the Commons occupy the same segment of memory , Modifying one member affects all other members .
The memory occupied by the structure is greater than or equal to the total memory occupied by all members （ There may be gaps between members ）, The memory occupied by the Commons is equal to the memory occupied by the longest member . Commons use memory overlay technology , Only one member's value can be saved at the same time , If you assign a value to a new member , It will override the value of the original member .

Commons are also a custom type , You can use it to create variables , for example ：

union data{
    int n;
    char ch;
    double f;
};
union data a, b, c;

 The above is to define the common body first , Then create variables , You can also create variables while defining a common body ：

union data{
    int n;
    char ch;
    double f;
} a, b, c;

 If you don't define new variables anymore , You can also omit the name of the common body ：

union{
    int n;
    char ch;
    double f;
} a, b, c;

 Shared body  data  in , member  f  Takes up the most memory , by  8  Bytes , therefore  data  Variable of type （ That is to say  a、b、c） It also takes up  8  Bytes of memory , Please see the following Demo ：

#include <stdio.h>

union data{
     int n;
    char ch;
    short m;
};

int main(){
    union data a;
    printf("%d, %d\n", sizeof(a), sizeof(union data) );
    a.n = 0x40;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.ch = '9';
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.m = 0x2059;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);
    a.n = 0x3E25AD54;
    printf("%X, %c, %hX\n", a.n, a.ch, a.m);

    return 0;
}

 Running results ：

4, 4
40, @, 40
39, 9, 39
2059, Y, 2059
3E25AD54, T, AD54

This code not only verifies the length of the common body , It also shows that members of the community will interact with each other , Changing the value of one member will affect other members .

To understand the output above , Find out how members interact , You have to understand the distribution of each member in memory . On the surface of the above data For example , The distribution of each member in memory is as follows ：

member n、ch、m In memory “ alignment ” To the end , Yes ch The assignment modifies the previous byte , Yes m The assignment changes the first two bytes , Yes n The assignment modifies all bytes . in other words ,ch、m Will affect n Part of the data , and n Will affect ch、m Full data .

The picture above is in the vast majority of PC Memory distribution on the computer , If it is 51 Single chip microcomputer , Things will be different ：

Why do different machines have different distributions ？ This is related to the storage mode of the machine , Big end and small end .