Union is a user-defined datatype similar to structs which allows to store different data types in the same memory location. Unions can be defined with many members, but only one member can contain a value at any given time.

How to define a Union

Union definition is similar to structure definition but you use union as the keyword.

union union_name {
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];  

Example

union mobile
{
    char model[30];
    char brand[30];
    int cost;   
};

How to create Union variables

Union variables can be declared in two ways.

1. Declaring variables seperately from its definition.

union union_name variable name;

2. Declaring variables along with definition(this method is not recommended)

union union_name {
   member definition;
   member definition;
   ...
   member definition;
} one or more union variables;  

Example

// Declaring variables seperately from its definition
union mobile m1, m2;

//Declaring variables along with definition
union mobile {
    char model[30];
    char brand[30];
    int cost; 
}m1, m2;

How to access members of a union

You can access the union member using variable_name.membername

Difference between Structures and Unions

• You can access all memebers of a structure at once but you can access only one member of a union at once.
• Size of a union variable will always be the size of its largest element where as size of a structure variable greater or equal to the sum of sizes of all its elements.
• Memory is shared by union members where as structure members are assigned with unique storage area.
• Several members can be initialized at once in structures where as first member of a union can be initialized.