What's the point of the union type?

Question

C++ / C#

What's the point of the union type? I know that it is well, I have not seen anywhere where it is used. Then what is its meaning?

Answer 1

What's the point of the union type? I know that it is well, I have not seen anywhere where it is used.

Everything that was written here in the answers about saving memory, about the rudiments of early languages ​​... and various other nonsense - this is all complete nonsense ... or very secondary and insignificant.
The main use of union is the imposition of different types of data on one common memory area, i.e. representation of one area of ​​memory as different data types. It comes from FORTRAN COMMON blocks and Pascal variant records.
It is used very widely in system programming - for example, the entire code of the Linux kernel is replete with unions.

Answer 2

In order to store data of different types in one volume.
For example, in x32 Int is 4 bytes, and Double is 8 bytes
Doing

union Some {
    int i;
    double a;
};

You can save space.

struct STRX {
   int j;
   Some v;
}

where j - will define the data type in the union Some
The size of the union will be the size of the type with the maximum size.
Where it is used: For data transmission over the network. You can have a constant packet size in STRX, but the data is different.

Answer 3

Sometimes you need to save memory in this way.
The most common use I've seen is structures in files.

Answer 4

You can make generic storage objects.
Example:

The code

#include <stdio.h>

struct a {
    int a, b, c;
};

struct b {
    double a, b, c, d;
};

struct c {
    char s[100];
};

struct d {
    int a, b, c;
    double n[10];
};

union x {
    struct a a;
    struct b b;
    struct c c;
    struct d d;    
};

typedef struct {
    enum { A, B, C, D } type;
    union x value;
} Obj;

int main(void)
{
    Obj objects[100], *pobj;
    int n;
    int i;

    objects[0].type = A;
    objects[0].value.a.a = 1;
    objects[0].value.a.b = 2;
    objects[0].value.a.c = 3;

    objects[1].type = B;
    objects[1].value.b.a = 1.5;
    objects[1].value.b.b = 2.5;
    objects[1].value.b.c = 3.5;
    objects[1].value.b.d = 4.5;

    objects[2].type = C;
    sprintf(objects[2].value.c.s, "abc");

    objects[3].type = A;
    objects[3].value.a.a = 4;
    objects[3].value.a.b = 5;
    objects[3].value.a.c = 6;

    n = 4;
    
    for (i = 0; i < n; i++) {
        pobj = objects + i;
        switch (pobj->type) {
        case A:
            printf("A: %d %d %d\n",
                   pobj->value.a.a,
                   pobj->value.a.b,
                   pobj->value.a.c);
            break;
        case B:
            printf("B: %g %g %g %g\n",
                   pobj->value.b.a,
                   pobj->value.b.b,
                   pobj->value.b.c,
                   pobj->value.b.d);
            break;
        case C:
            printf("C: %s\n", pobj->value.c.s);
            break;
        case D:
            printf("D: %g\n", pobj->value.d.n[0]);
            break;
        }
    }

    return 0;
}

Conclusion

[[email protected] c]$ .ansi t.c -o t
[[email protected] c]$ ./t
A: 1 2 3
B: 1.5 2.5 3.5 4.5
C: abc
A: 4 5 6
[[email protected] c]$

I won't give you a direct answer, but I'll give you something to think about. It's a paradox, but programming languages ​​still implement the goto operator, despite the fact that it has been marked as obsolete / deprecated for 10 years and generally not taken into hands. What for? perhaps for backward compatibility, perhaps in the bowels of the implementation it is used somewhere. Don't use? Well, then don't use it. What's the problem then? Yes, the Pareto principle has not been canceled - 80% of users use only 20% of the functionality.

This is an attempt to implement algebraic data types .

Answer 5

I won't give you a direct answer, but I'll give you something to think about. It's a paradox, but programming languages ​​still implement the goto operator, despite the fact that it has been marked as obsolete / deprecated for 10 years and generally not taken into hands. What for? perhaps for backward compatibility, perhaps in the bowels of the implementation it is used somewhere. Don't use? Well, then don't use it. What's the problem then? Yes, the Pareto principle has not been canceled - 80% of users use only 20% of the functionality.

Answer 6

This is an attempt to implement algebraic data types .