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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.