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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Top 10 most common hard skills listed on resumes...
Date: Sun, 08 Sep 2024 12:19:18 -0700
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Waldek Hebisch <antispam@fricas.org> writes:
> Bart <bc@freeuk.com> wrote:
>> 
>> But the feature (using them in lvalue contexts) was rarely used.
>> 
>> (** This is:
>> 
>>     (a, b, c) = 0;
>> 
>> in C, which is illegal. But put in explicit pointers, and it's suddenly 
>> fine:
>> 
>>     *(a, b, &c) = 0;
>> 
>> So why can't the compiler do that?)

A C compiler doesn't do that because the language doesn't allow it.
An expression whose top-level operator is a comma operator is not
an lvalue in C, even if the operands are lvalues.  It could have been,
but it isn't.

> The second really is
>
>   (a, b, c = 0);
>
> I do not know what you expect writing '(a, b, c) = 0', but the above
> C meaning probably is not what you want.

I think Bart was talking about a hypothetical feature that allows comma
expressions to be lvalues.  The comma operator evaluates both its
operands and yields the value of the right operand.  So for example:

    (a, b) = 0;

which is a constraint violation in current C, the LHS (a, b) would be
treated as an lvalue that discards a and designates b, which then
becomes the target of the assignment.

It's not an insane idea, and it wouldn't be difficult to specify its
semantics consistently.  In fact C++ supports conditional and comma
expressions as lvalues; `(a, b) = 0;` is a constraint violation in C,
but valid in C++.  I'm not sure how useful it is.

> I use language that allows:
>
>   (a, b, c) := (b, c, a);
>
> that is if there are 3 things on left hand side, then there must
> be 3 things on the right hand side.

That's not a comma operator, and I don't think it's relevant to what
Bart is talking about.  The comma symbol is used in a lot of different
contexts.

Bart, please jump in if I've misunderstood what you meant.

[...]

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */