Re: unions

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From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c++
Subject	Re: unions
Date	Sat, 04 Jun 2011 17:38:47 +1200
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On 06/ 4/11 05:26 PM, Ruben Safir wrote:
> There are some things, that as I learned to program, I learned them,
> decided they suck and would never use them, and then put them away.
> unions are one of these things (static was another and complex #define
> yet another).
>
> But I took this employment exam this week and 10% of the questions
> involved unions.  As I understood unions, union sub-objects just step on
> each other in binary in the allocated space where they exist (talk about
> a snake pit for potential bugs).  Am I missing something about them?  I
> started to have deep doubts about what I thought I knew.

That's about right.  You could view a union as a struct with all the 
members having an offset of zero.

We don't use them in C++ as much as in C.

> union {
> 	double x;
> 	char a[10];
> } myunion;
>
> myunion.a[0] = 'A';
> myunion.a[1] = 'A';
> myunion.a[2] = 'A';
> myunion.a[3] = 'A';
>
> std::cout<<  myunion.x<<  std::end
> is going to spit out some binary translation of a double with 4 bytes of
> 65 dec, whatever that is.

Maybe, the result is undefined.

-- 
Ian Collins

Thread

unions Ruben Safir <ruben@mrbrklyn.com> - 2011-06-04 05:26 +0000
  Re: unions Ian Collins <ian-news@hotmail.com> - 2011-06-04 17:38 +1200
    Re: unions Ruben Safir <ruben@mrbrklyn.com> - 2011-06-04 05:45 +0000
      Re: unions Ian Collins <ian-news@hotmail.com> - 2011-06-04 18:04 +1200
        Re: unions Ruben Safir <ruben@mrbrklyn.com> - 2011-06-04 06:13 +0000
  Re: unions Öö Tiib <ootiib@hot.ee> - 2011-06-04 04:54 -0700
  Re: unions Edek <edek.pienkowski@gmail.com> - 2011-06-04 19:59 +0200
    Re: unions Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-04 20:13 +0200
      Re: unions Edek <edek.pienkowski@gmail.com> - 2011-06-04 22:34 +0200
        Re: unions Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-05 07:29 +0200
  Re: unions Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-06-09 09:19 +0000
  Re: unions Keith <kbelder@gmail.com> - 2011-06-09 11:17 -0700
    Re: unions Victor Bazarov <v.bazarov@comcast.invalid> - 2011-06-09 14:29 -0400
    Re: unions Edek <edek.pienkowski@gmail.com> - 2011-06-10 13:16 +0200

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