Union
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Introduction
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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.
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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
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Union-Definition and Declaration
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Unions, like structures, contain
members whose individual data types may differ from one another.
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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.
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Thus, unions are used to conserve
memory.
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They are useful for applications
involving multiple members, where values need not be assigned to all of the
members at any one time.
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Within a union, the bookkeeping
required to store members whose data types are different (having different
memory requirements) is handled automatically to the compiler.
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However, the user must keep track of
what type of information is stored at any given time.
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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:
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Union tag
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{
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Member 1;
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Member 2;
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…..
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member n;
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};
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Where union is required keyword and
the other terms have the same meaning as in a structure definition.
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Individual union variables can then
be declared as:
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storage-class union tag variable
1, variable 2, . . . , variable n;
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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.
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The two declarations may be combined,
just as we did with structures. Thus, we can write Storage-class
union tag
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{
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Member 1;
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Member 2;
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. . . . .
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member n;
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}
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The tag is optional in this type of
declaration.
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Notice that the union and structure declarations are
external to the program functions, but the structure variable is defined
locally within each function.
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Accessing a union member
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To access a union member, you can use
the same syntax that you use for structure members.
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Example:
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code.m, code.x etc.
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During execution, we should make sure
that the value of accessing member is currently stored.
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Initialization of Union variable
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A union variable can be initialized ,
provided its storage class is either external or static.
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Only one member of a union can be
assigned a value at any one time.
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The initialization value is assigned
to the first member within the union.
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Uses of Union
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Union, like structure, contain members whose
individual data type may differ to each other.
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But the members that compose a union share
the same storage area within the computer's memory, whereas each member
within a structure is assigned its own unique storage area.
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Thus, union are used
to conserve memory.
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1. Unions are useful
for application involving multiple members, where value need not to be
assigned to all of the members at any one time.
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2. Unions are useful
whenever there is a requirement to access the same memory location in more
than one way. etc.
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