A union is a variable which may hold (at different times) objects of different sizes and types. C uses the union statement to create unions, for example:
defines a union called number and an instance of it called anumber. number is a union tag and acts in the same way as a tag for a structure.
Members can be accessed in the following way:
This clearly displays the value of longnumber.
When the C compiler is allocating memory for unions it will always reserve enough room for the largest member (in the above example this is 8 bytes for the double).
In order that the program can keep track of the type of union variable being used at a given time it is common to have a structure (with union embedded in it) and a variable which flags the union type:
An example is:
This example defines a base union aircraft which may either be jet, helicopter, or cargoplane.
In the an_aircraft structure there is a kind member which indicates which structure is being held at the time.