Unions

#unions#


C Unions are essentially the same as C Structures, except that instead of containing multiple variables each with their own memory a Union allows for multiple names to the same variable. These names can treat the memory as different types (and the size of the union will be the size of the largest type, + any padding the compiler might decide to give it)
So if you wanted to be able to read a variable's memory in different ways, for example read an integer one byte at a time, you could have something like this:

union intParts {
  int theInt;
  char bytes[sizeof(int)];
}; 

Allowing you to look at each byte individually without casting a pointer and using pointer arithmetic:


union intParts parts;
parts.theInt = 5968145; // arbitrary number > 255 (1 byte)

printf("The int is %i\nThe bytes are [%i, %i, %i, %i]\n",
parts.theInt, parts.bytes[0], parts.bytes[1], parts.bytes[2], parts.bytes[3]);

// vs

int theInt = parts.theInt;
printf("The int is %i\nThe bytes are [%i, %i, %i, %i]\n",
theInt, *((char*)&theInt+0), *((char*)&theInt+1), *((char*)&theInt+2), *((char*)&theInt+3));

// or with array syntax which can be a tiny bit nicer sometimes

printf("The int is %i\nThe bytes are [%i, %i, %i, %i]\n",
    theInt, ((char*)&theInt)[0], ((char*)&theInt)[1], ((char*)&theInt)[2], ((char*)&theInt)[3]);

 

Combining this with a structure allows you to 
create a "tagged" union which can be used to store multiple different 
types, one at a time.


For example, you might have a "number" struct, but you don't want to use something like this:


struct operator {
    int intNum;
    float floatNum;
    int type;
    double doubleNum;
};