Union

è    Structure data type is so defined that is able to accept heterogeneous(mixed) data types. A union is also similar to structure. union is a variable which may hold objects of different data types and size.

è    The important difference between structure and union is that structure is a group of variable of different types, while union is a single variable whose type is varying among the type defined as members of union . Union variable is allocated the memory of the largest type and then compiler keeps track of size and alignment according to  the last value of particular data type.

è                union can be created  as follows:

union tag_name

{

          type member1

          type member2

          .....

          .....

}

Here, the keyword union is used to create union. The tag_name specifies the name of union. member1,member2 ... specifies the union member of particular type.

Ex, union data

          {

                   char ch;

                   int i;

                   float f;

          }u;

In above example the union name is data which contains three members of character type, integer type and float type.

The u is the variable whose type can be character, integer or float but at a time any one only. The largest type is float and hence memory allocated to union variable u is equal to size of float.

The member of a union data type shares a same memory. An union member accessed using  . dot operator such as

                                        union_var_name.member name

Ex;

                                        u.i=10;

                                        printf(“\n i= %d”,u.i);

Here, the value stored in union is of integer type hence the access must be of type integer.when try to print u.c or u.f , response is garbage.If use

                                        u.f=10.5;

here, it overwrites the previous integer .As last storage is of float the access should be of type float only.