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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Union type punning in C++
Date: Tue, 21 Jan 2020 02:03:09 -0800
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Xref: csiph.com comp.lang.c++:71480

Tiib writes:

> On Tuesday, 14 January 2020 17:30:46 UTC+2, Tim Rentsch  wrote:
>
>> Tiib writes:
>>
>>> On Monday, 13 January 2020 19:02:00 UTC+2, Tim Rentsch  wrote:
>>
>> [context:  this code
>>
>>     struct base {
>>          int p0;
>>     };
>>
>>     struct object_0 {
>>          base b;
>>          int p0;
>>     };
>>
>> ]
>>
>>>> The first member of struct base is of type 'int'.  The first member of
>>>> struct object_0 is of type 'base'.  The two types, 'int' and 'base',
>>>> are not layout compatible.  Do you agree or disagree?
>>>
>>> I disagree.  The address of base is required to be address of int as
>>> well as first element of base so these are layout compatible.
>>
>> In N4659, section 6.9 paragraph 11 defines the term 'layout
>> compatible'.  The definition says (leaving out the cv's) that two
>> types T1 and T2 are layout compatible if
>>
>>    (a) T1 and T2 are the same type,
>>
>>    (b) T1 and T2 are layout-compatible enumerations, or
>>
>>    (c) T1 and T2 are layout-compatible standard-layout class types.
>>
>> When T1 is 'int' and T2 is 'base' (aka 'struct base'), which of these
>> categories do you think applies?
>
> T1 is also 'base' so (a) applies.

No, it isn't.  It appears that either you haven't been listening
or you aren't thinking clearly.