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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: lvalue types
Date: Sat, 03 Mar 2018 13:01:07 -0800
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Tim Rentsch <txr@alumni.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
>
>> [When using . or ->, what is "accessed"?]
>>
>> Let me make the point more succinctly.
>>
>>     struct s { int m; };
>>     struct s obj;
>>     obj.m = 42;
>>
>> obj.m is an lvalue of type int.  The assignment accesses (specifically
>> modifies) the object obj.m, which is of type int;  that's perfectly fine
>> under 6.5p7.  It also accesses (i.e., modifies) the object obj, which
>> is of type struct s.
>
> I don't subscribe to this view, more specifically the last
> sentence.  Consider a union rather than a struct:
[snip]

Let's consider the original struct.

N1570 3.1 defines "access" as

    <execution-time action> to read or modify the value of an object

Wouldn't you agree that `obj.m = 42;` modifies the value of obj?
Doesn't it therefore access obj?

Are you saying that
    obj.m = 42;
only modifies obj.m, but
    obj = (struct s){.obj = 42};
(which has an equivalent effect) modifies both obj and obj.m?

[...]

>>  N1570 6.5p7 says that:
>>
>>     An object shall have its stored value accessed only by an lvalue
>>     expression that has one of the following types:
>>
>> followed by 6 bullet points, none of which allow an object of type
>> struct s to be accessed by an lvalue of type int.
>>
>> It's obvious that the assignment is meant to be well defined.  I'd say
>> this is a simple oversight in the wording of the standard (unless
>> someone can come up with another interpretation).
>
> It's highly unlikely it's just an oversight.  Among other things,
> this clause
>
>     an aggregate or union type that includes one of the
>     aforementioned types among its members (including,
>     recursively, a member of a subaggregate or contained union)
>
> has been present in the Standard since at least 1989.

Yes, which means that modifying a struct object modifies its members.
I don't understand why the reverse would not also be true.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"