Union( Introduction )

Introduction

Union are derived data types, the way structure are.
Though, unions and structures look alike, and there is a fundamental difference.

While structure enables you to create a number of different
variables stored in difference places in memory, unions enable you to treat the same space as a number of different variables

Union-Definition and Declaration

Unions, like structures, contain members whose individual
data types may differ from one another.

However, the members within a union all share the some
storage space within the computer's memory, whereas each
member within a structure is assigned its own unique
storage area.

Thus, unions are used to conserve memory.

They are useful for applications involving multiple members,
where values need not be assigned to all of the members
at any one time.

Within a union, the bookkeeping required to store members
whose data types are different (having different memory requirements)
is handled automatically to the compiler.

However, the user must keep track of what type of information
is stored at any given time.

An attempt to access the wrong type of information will
produce meaningless results. In general terms, the
composition of a union may be defined as:

Union tag
{
Member 1;
Member 2;
…..
member n;
};

Where union is required keyword and the other terms have the
same meaning as in a structure definition.

Individual union variables can then be declared as:
storage-class union tag variable 1, variable 2, . . . , variable n;

Where storage-class is an optional storage
class specified, union is a required keyword, tag is
the name that appears in the union definition,
and variable 1, variable 2, . . . , variable n are union.

The two declarations may be combined, just as we did
with structures. Thus, we can write Storage-class union tag

{
Member 1;
Member 2;
. . . . .
member n;
}

The tag is optional in this type of declaration.

Notice that the union and structure declarations are
external to the program functions, but the structure
variable is defined locally within each function.