C Unions
A union looks a lot like a structure, but with one big difference: all of its members share the same block of memory. That means a union can hold only one of its members at any given time. Unions are used when you want a single variable to store different kinds of data at different moments, while saving memory.
How a union differs from a struct
In a struct, every member gets its own space, so the total size is the sum of all members. In a union, the members overlap, so the total size is just big enough for the largest member. Writing to one member overwrites the others.
Defining and using a union
#include <stdio.h>
union Value {
int i;
float f;
char c;
};
int main() {
union Value v;
v.i = 42;
printf("As int: %d\n", v.i);
v.f = 3.14f; // overwrites the same memory
printf("As float: %.2f\n", v.f);
return 0;
}After v.f = 3.14f, reading v.i would give a meaningless number, because the bits now represent a float. You should only read the member you most recently wrote.
Union size
Because all members share storage, the size of a union equals the size of its largest member (possibly rounded up for alignment). You can confirm this with the sizeof operator.
Checking the size of a union
#include <stdio.h>
union Mixed {
char letter; // 1 byte
int number; // 4 bytes
double amount; // 8 bytes
};
int main() {
printf("Size of union: %lu bytes\n", sizeof(union Mixed));
return 0;
}
