(C++) union

 

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)

 

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#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:

 

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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

 

1. 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.'
2. 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'