Unions
·
Unions concept and syntax is
the same as structures.
·
There is a difference between
structure and union in terms of storage.
·
In the structure, each member
has its own storage location, where as all the members of the union use the
same location.
·
A union may contain many
members of different types, it can handle only one member at a time.
·
Like the structures, a union
can be declared using the keyword union as follows.
union item
{
int m;
float x;
char c;
} code;
·
This declares a variable code
of type union item.
·
The union contains three
members, each with a different data type.
·
We can use only one of them at
a time.
·
This is due to the fact that
only one location is allocated for a union variable, irrespective of its size.
Example:
The member 'x' in the union 'code'
·
The figure shows how all the
three variables share the same address.
·
This assumes that a float
variable requires 4 bytes of storage.
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·
To access union members, we can
use the same syntax that we use for structure members, i.e.
code.m
code.x
code.c are
all valid member variables.
·
During accessing, we should
make sure that we are accessing the member whose value is currently stored.
Example:
code. m=189;
code.x=759.36;
printf("%d",
code.m);
would produce
inaccurate ouput (machine dependent)
·
Only one member uses the memory
allocated in a union variable.
·
When a different member is
assigned a new value, the new value takes the place of a previous member value.
·
Unions are used in all places
where the structure is allowed.
·
Unions may be initialized when
the variable is declared. But unlike structures, it can be initialized only
with a value of the same type as the first union member.
Example:
union item
abc={100}; // valid
union item abc={10.75};
// invalid.
·
This is because the type of the
first member is int.
·
Other members can be
initialized by either assigning values or reading from the console.
