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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Don't be fooled by cpp.sh
Date: Fri, 10 Jan 2020 21:26:38 -0800
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Daniel <danielaparker@gmail.com> writes:

> On Sunday, December 29, 2019 at 1:30:50 PM UTC-5, bol...@nowhere.org wrote:
>
>> The whole point of a union is not to save memory, its to write as
>> one type and read as another
>
> Not according to K&R, 1st edition.  K&R write
>
> "This is the purpose of a union ? to provide a single variable which
> can legitimately hold any one of several types.
>
> union u_tag {
>
> int ival;
> float fval;
> char *pval;
> } uval;
>
> The variable uval will be large enough to hold the largest of the
> three types, regardless of the machine it is compiled on ? the code
> is independent of hardware characteristics.  Any one of these types
> may be assigned to uval and then used in expressions, so long as the
> usage is consistent:  the type retrieved must be the type most
> recently stored.  It is the responsibility of the programmer to keep
> track of what type is currently stored in a union;  the results are
> machine dependent if something is stored as one type and extracted
> as another."

As far as how unions behave goes, what this says is basically the
same as what C89/C90 says, which in turn has the same intended
meaning as the more complete descriptions in later C standards.

Note that I'm not saying anything about the main or intended purpose
of unions.  Clearly though K&R recognized that unions could be used
for type punning (as it is called today).