union is a C++ keyword to declare a struct that uses the same memory for all elements.

In the example below, a union is created consisting of a double and an int. When the int is modified, the double is modified as well (and vice versa).

Download the Qt Creator project 'CppUnion' (zip)


#include <boost/static_assert.hpp>

#include <iostream>



struct Struct

{

  double d;

  int i;

};



union Union

{

  double d;

  int i;

};



int main()

{

  //Check that a unions' size is

  //smaller than a struct's

  BOOST_STATIC_ASSERT(sizeof(Union)<sizeof(Struct));



  //Create a clean union

  Union u;

  u.i = 0;

  u.d = 0;



  u.i = 1000;

  std::cout

    << "int   : " << u.i << '\n'

    << "double: " << u.d << '\n';

  u.d = 1000.0;

  std::cout

    << "int   : " << u.i << '\n'

    << "double: " << u.d << '\n';



}

Screen output:


int   : 1000
double: 4.94066e-321
int   : 0
double: 1000

Consider never using unions [1]. Use unions to save space [2]. Never use unions for type conversion [2].

References

Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 153 (MISRA Rule 110, Revised): 'Unions shall not be used.'
Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 8.5. Advice. page 224: '[4] Use unions to save space (represent alternatives and never for type conversion'

Go back to Richel Bilderbeek's C++ page.

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(C++) union

References