?: as an lvalue

Dave Hansen wrote:
>
> I believe the syntax is legal.

Legal where? In C? No, it is not.

> It just doesn't do what the OP wanted
> it to.

It doesn't do anything. It is illegal in C.

> Consider that it parses as
>
>    (1>0)?(aa):(b=10);

But it doesn't parse like that in C.

> Change it to 1<0?aa:b=10; and see the result...

The result will be the same: a diagnostic message triggered by the
attempt to assign to a non-lvalue.

--
Best regards,
Andrey Tarasevich

Andrey wrote:
) Richard wrote:
) > ...
)> But (c?x:y)=v;
)> I dont really know what to say.
)
) Is there any reason why you believe that the property of "being an
) lvalue" should be necessarily lost in the process of selection from two
) lvalues of the same type?
)
) I mean I'm OK personally with the way it works in C. I just like to know
) what is it exactly in '(c?x:y)=v' that triggers a "I don't really know
) what to say" reaction from some people.

Well then why not also make it possible for functions (that return
pointers) to be lvalues ?

returnspointertostruct(foo)->bar = baz;

Or is that already legal ?


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

Bartc wrote, On 31/03/08 18:45:
> "Richard" <devr_@gmail.com> wrote in message
> news:fsr52k$1id$1@registered.motzarella.org... 
> 
> ... 
>
> You can argue the same way about using a?b:c as an rvalue.
> 
>
> a:      You can write a = x without writing *(&a) = x

You can't do 3 = x

> a.b:    You can write a.b = x without writing *(&a + offsetof b) = x

You can't do 5.b or a.5

>           (I think offsetof is more complex than that)
> a->b:   You can write a->b = x without writing *(&a + offsetof b) = x

You can't do 5->b or b->5

> a[b]:   You can write a[b] = x without writing *(a+b) = x

Unless b is a pointer or array name you can't do 5[b]

> a?b:c:  You CAN'T write a?b:c = x without writing *(a ? &b : &c) = x

You can do a?1:2

> So the mystery is why the compiler gives special dispensation to those oth
er
> forms when an lvalue is expected, but not the ?: form.
>
> Someone will say, for the same reason as a+b can't usually be an lvalue, b
ut
> I think ?: is a little different.

I think you are wrong. Look at the fact that all of the other operators
you mention *require* at least one of the operands to be an object but
neither + nor ?: require and operand to be an object. Then you should
see why it is natural for a?b:c = x to be wrong.
--
Flash Gordon

Richard wrote: 
>
> because its not a macro? it returns a value. I dont know the legalise
> words but it seems "obvious" enough to me, but again it might be because
> I am tainted.

Well, unary '*' operator is also not a macro. Yet it evaluates to an
lvalue. Same for '[]' operator (by definition). Do you find this strange
as well?

--
Best regards,
Andrey Tarasevich

Willem wrote:
> ) > ...
> )> But (c?x:y)=v;
> )> I dont really know what to say.
> )
> ) Is there any reason why you believe that the property of "being an
> ) lvalue" should be necessarily lost in the process of selection from two
> ) lvalues of the same type?
> )
> ) I mean I'm OK personally with the way it works in C. I just like to know
> ) what is it exactly in '(c?x:y)=v' that triggers a "I don't really know
> ) what to say" reaction from some people.
>
> Well then why not also make it possible for functions (that return
>  pointers) to be lvalues ?
>
>   returnspointertostruct(foo)->bar = baz;
>
> Or is that already legal ?

This is perfectly legal. It's just that has nothing to do with the
function. Function still returns a non-lvalue. The left-hand side
expression though

(non-lvalue pointer)->bar

is an lvalue. This why you can assign to it.

What's illegal is this

returnsstruct(foo).bar = baz;

i.e. function returns a 'struct' in this case .

--
Best regards,
Andrey Tarasevich

In article <fsrc0v$4ov$3@registered.motzarella.org>,
Richard  <devr_@gmail.com> wrote:
 

>because its not a macro? it returns a value. I dont know the legalise
>words but it seems "obvious" enough to me, but again it might be because
>I am tainted.

Some other languages allow this.  For example, I think Algol 68 allows

IF a THEN b ELSE c FI := d;

In C the number of contexts where an lvalue doesn't decay into an rvalue
is very small, but I don't see anything inherently un-C-like about this
case.

-- Richard



--
:wq

Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> Richard wrote: 
>
> Well, unary '*' operator is also not a macro. Yet it evaluates to an
> lvalue. Same for '[]' operator (by definition). Do you find this
> strange as well?

I think you have lost the track with all due respect. His original did
produce an lvalue but a value.

The "*" made it then an lvalue in the other case.

I dont really know what we are aguing.

There appears to be some sort of push for ?: to return an lvalue but it
doesnt and never did.

richard@cogsci.ed.ac.uk (Richard Tobin) writes:

> In article <fsrc0v$4ov$3@registered.motzarella.org>,
> Richard  <devr_@gmail.com> wrote:
> 
> 
>
> Some other languages allow this.  For example, I think Algol 68 allows
>
>   IF a THEN b ELSE c FI := d;
>
> In C the number of contexts where an lvalue doesn't decay into an rvalue
> is very small, but I don't see anything inherently un-C-like about this
> case.
>
> -- Richard

And he can do it in C using pointers and *. I dont see  the problem or
the confusion here as far as the C language goes to be honest.

In article <fsrdh6$ikl$1@registered.motzarella.org>,
Richard  <devr_@gmail.com> wrote:
 

>I think you have lost the track with all due respect. His original did
>produce an lvalue but a value.

I think the point was that * is an example of an operator that produces
an lvalue, so it's not necessarily unreasonable for the ?: operator to.

-- Richard
--
:wq

Richard wrote: 
>
> I think you have lost the track with all due respect. His original did
> produce an lvalue but a value.
>
> The "*" made it then an lvalue in the other case.
>
> I dont really know what we are aguing.

I'm not really arguing. The way I interpreted your responses, it seemed
that when someone said that in C++ '?:' would return an lvalue in this
case, you essentially made it clear that you find it unnatural and/or
illogical (again, the way I interpreted your responses). I just want to
know what is it exactly that you find unnatural and/or illogical.

--
Best regards,
Andrey Tarasevich