Unions are like structures, but they allow several datatypes to use the same memory location.
union union_tag{
datatype1 attr1;
datatype2 attr2;
} some_union_vars...;Dynamic Size
https://www.youtube.com/watch?v=ZMzdrEYKyFQ Usually when structures are declared, the sum of all of the size of its attributes is allocated to the stack/heap. Unions will only allocate the largest attribute. all attributes will then share this allocated memory space.
Polymorphism
Because all elements share the same memory, if you wanted to use a datatype as another datatype, that is possible too
#include <stdio.h>
#include <string.h>
union myunion{
int layers;
char type[20];
};
int main(){
union myunion = {5};
strcpy(myunion, "green");
printf("%d", myunion.type); // string as int. will be 0000
return 0;
}