Union
A union allows you a way to look at the same data with different types, or to use the same data with different names. Unions are similar to structures. A union is declared and used in the same ways that a structure is.
A union differs from a structure in that only one of its members can be used at a time. The reason for this is simple. All the members of a union occupy the same area of memory. They are laid on top of each other.
Unions are defined and declared in the same fashion as structures. The only difference in the declarations is that the keyword union is used instead of struct. To define a simple union of a char variable and an integer variable, you would write the following:
union shared {
char c;
int i;
};
This union, shared, can be used to create instances of a union that can hold either a character value c or an integer value i. This is an OR condition. Unlike a structure that would hold both values, the union can hold only one value at a time.
A union can be initialized on its declaration. Because only one member can be used at a time and only one can be initialized. To avoid confusion, only the first member of the union can be initialized. The following code shows an instance of the shared union being declared and initialized:
union shared generic_variable = {`@'};
Notice that the generic_variable union was initialized just as the first member of a structure would be initialized.
Individual union members can be used in the same way that structure members can be used by using the member operator (.). However, there is an important difference in accessing union members.
Only one union member should be accessed at a time. Because a union stores its members on top of each other, it's important to access only one member at a time.
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