Unions In Cpp

Unions:

Unions allow one same portion of memory to be accessed as different data types, since all of them are in fact the same location in memory. Its declaration and use is similar to the one of structures but its functionality is totally different:

Syntax
union union_name {
	member_type1 member_name1;
	member_type2 member_name2;
	member_type3 member_name3;
	.
	.
	} object_names;

All the elements of the union declaration occupy the same physical space in memory. Its size is the one of the greatest element of the declaration.

For example :

CODE/PROGRAM/EXAMPLE
union mytypes_t {
	char c;
	int i;
	float f;
	} mytypes;

defines three elements:

CODE/PROGRAM/EXAMPLE
mytypes.c
mytypes.i
mytypes.f

each one with a different data type. Since all of them are referring to the same location in memory, the modification of one of the elements will affect the value of all of them. We cannot store different values in them independent of each other.

One of the uses a union may have is to unite an elementary type with an array or structures of smaller elements.

For example :

CODE/PROGRAM/EXAMPLE
union mix_t {
	long l;
	struct {
	short hi;
	short lo;
	} s;
	char c[4];
	} mix;

defines three names that allow us to access the same group of 4 bytes: mix.l, mix.s and mix.c and which we can use according to how we want to access these bytes, as if they were a single long-type data, as if they were two short elements or as an array of char elements, respectively.

I have mixed types, arrays and structures in the union so that you can see the different ways that we can access the data. For a little-endian system (most PC platforms), this union could be represented as:

union in c++

The exact alignment and order of the members of a union in memory is platform dependant. Therefore be aware of possible portability issues with this type of use.

Anonymous unions:

In C++ we have the option to declare anonymous unions. If we declare a union without any name, the union will be anonymous and we will be able to access its members directly by their member names.

For example, look at the difference between these two structure declarations:

structure with regular union structure with anonymous union
struct { char title[50]; char author[50]; union { float dollars; int yens; } price; } book; struct { char title[50]; char author[50]; union { float dollars; int yens; }; } book;

The only difference between the two pieces of code is that in the first one we have given a name to the union (price) and in the second one we have not. The difference is seen when we access the members dollars and yens of an object of this type. For an object of the first type, it would be:

CODE/PROGRAM/EXAMPLE
book.price.dollars
book.price.yens

whereas for an object of the second type, it would be:

CODE/PROGRAM/EXAMPLE
book.dollars
book.yens

Once again I remind you that because it is a union and not a struct, the members dollars and yens occupy the same physical space in the memory so they cannot be used to store two different values simultaneously.

You can set a value for price in dollars or in yens, but not in both.

Enumerations (enum):

Enumerations create new data types to contain something different that is not limited to the values fundamental data types may take. Its form is the following:

Syntax
enum enumeration_name {
	value1,
	value2,
	value3,
	.
	.
	} object_names;

For example, we could create a new type of variable called color to store colors with the following declaration:

Syntax
enum colors_t {black, blue, green, cyan, red, purple, yellow, white};

Notice that we do not include any fundamental data type in the declaration. To say it somehow, we have created a whole new data type from scratch without basing it on any other existing type.

The possible values that variables of this new type color_t may take are the new constant values included within braces.

For example, once the colors_t enumeration is declared the following expressions will be valid:

CODE/PROGRAM/EXAMPLE
colors_t mycolor;
mycolor = blue;
if (mycolor == green) mycolor = red;

Enumerations are type compatible with numeric variables, so their constants are always assigned an integer numerical value internally. If it is not specified, the integer value equivalent to the first possible value is equivalent to 0 and the following ones follow a +1 progression.

Thus, in our data type colors_t that we have defined above, black would be equivalent to 0, blue would be equivalent to 1, green to 2, and so on.

We can explicitly specify an integer value for any of the constant values that our enumerated type can take. If the constant value that follows it is not given an integer value, it is automatically assumed the same value as the previous one plus one.

For example :

CODE/PROGRAM/EXAMPLE
enum months_t { january=1, february, march, april,
may, june, july, august,
september, october, november, december} y2k;

In this case, variable y2k of enumerated type months_t can contain any of the 12 possible values that go from january to december and that are equivalent to values between 1 and 12 (not between 0 and 11, since we have made january equal to 1).