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What are Unions (C programming)

Structures are used to hold a whole lot of different pieces of data in a collection - by element name not number. Sometimes, you •[99]ll only want to hold some elements of your structure in specific circumstances - a good example is where you have a whole lot of similar things you •[99]re describing, but only a limited range of elements applied to each.

A very good example in real  •[99]system •[99] programming is an X Windows event - each has a time stamp and a sequence number, and an event type, and each has a target window. But only some of them have X and Y co-ordinates, only some of them have keypresses and statuses, and so on. One possible way of holding such events would be a sparsely populated structure, but in practise this would be very wasteful of memory - so a union is used.
(written 2008-06-08, updated 2008-06-10)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C209 - C and C based languages - Structures and Unions
  [3386] Adding the pieces together to make a complete language - C - (2011-08-11)
  [3145] Structures v Structure Pointers in C. How, which, why. - (2011-01-25)
  [3122] When is a program complete? - (2011-01-06)
  [2573] C Structs - what, how and why - (2010-01-13)
  [1584] Using Structs and Unions together effectively in C - (2008-03-21)
  [1572] C - structs and unions, C++ classes and polymorphism - (2008-03-13)
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)


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