?: as an lvalue

[Rahul]

Hi Everyone,

I have the following piece of code, and i expected an error, however
i don't get an error,

int main()
{
int aa=0,b=0;
1>0?aa:b = 10;
printf("value %d %d\n",aa,b);
return(0);
}

and output is value 0 0

Thanks in advance ! ! !

[Keith Thompson]

Harald van Dijk <tr*****@gmail.comwrites:

On Wed, 02 Apr 2008 15:39:32 -0400, lawrence.jones wrote:

>Keith Thompson <ks***@mib.orgwrote:

>>Undefined behavior is illegal?

Well, the C Police aren't going to come and arrest you, but it's not
valid, which is what most people mean by "legal".

Most people don't consider the things illegal that the police turn a
blind eye towards. Undefined behaviour is what happens when your program
doesn't play by C's rules, but at the same time, C's rules tell your
compiler not to arrest you.

Actually, C's rules specifically *don't* tell the compiler not to
arrest you. Printing an error message and terminating, either at
compile time or at run time, is well within the (nonexistent) bounds
of undefined behavior.

Personally, I tend to think of anything that requires a diagnostic (a
syntax error or constraint violation) as "illegal" -- and even then it
can be "legal" in the sense that the implementation is still allowed
to produce an executable.

If everybody else thinks of UB as "illegal", I suppose I'll go along
with the consensus, but I don't think I'm the only one who finds the
usage ambiguous.

--
Keith Thompson (The_Other_Keith) <ks***@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Richard Heathfield]

Keith Thompson said:

<snip>

If everybody else thinks of UB as "illegal", I suppose I'll go along
with the consensus, but I don't think I'm the only one who finds the
usage ambiguous.

Is it illegal to use the extensions provided by an implementation? Is it
illegal to call functions in a third-party library? Is it illegal to talk
to another computer over a network?

It is one thing for the Standard to decline to define what will happen if
you do <foo>, but quite another to forbid you from doing it.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Andrey Tarasevich]

la************@siemens.com wrote:

Andrey Tarasevich <an**************@hotmail.comwrote:

>In C99 array-to-pointer conversion is applied to non-lvalue arrays,
meaning that the above code becomes legal. An interesting side effect of
this change is that in C99 the following is legal as well

foo().a[1] = 5;

No, it's not:

If an attempt is made to modify the result of a function call or
to access it after the next sequence point, the behavior is
undefined.

I was talking about the syntactical legality, meaning that the above
code doesn't violate any syntax rules or constraints of C99, as opposed
to those of C89/90. This is, I believe, what the word "legal" usually
means in the context of C language (due to lack of more tightly defined
terms like ill-/well-formed in C++).

--
Best regards,
Andrey Tarasevich

[Harald van =?UTF-8?b?RMSzaw==?=]

On Wed, 02 Apr 2008 19:03:44 -0700, Keith Thompson wrote:

Harald van Dijk <tr*****@gmail.comwrites:

>On Wed, 02 Apr 2008 15:39:32 -0400, lawrence.jones wrote:

>>Keith Thompson <ks***@mib.orgwrote:
Undefined behavior is illegal?

Well, the C Police aren't going to come and arrest you, but it's not
valid, which is what most people mean by "legal".

Most people don't consider the things illegal that the police turn a
blind eye towards. Undefined behaviour is what happens when your
program doesn't play by C's rules, but at the same time, C's rules tell
your compiler not to arrest you.

Actually, C's rules specifically *don't* tell the compiler not to arrest
you. Printing an error message and terminating, either at compile time
or at run time, is well within the (nonexistent) bounds of undefined
behavior.

I stand by my previous statement.

void f(){}
void g(){if(0)f(1);}

This has been declared a valid translation unit, and it may appear in
strictly conforming programs. A compiler is not allowed to reject it, as
long as the call to f is never executed, and in the general case, it's
not possible for the compiler to determine whether such a call would ever
be executed.

[Keith Thompson]

Harald van Dijk <tr*****@gmail.comwrites:

On Wed, 02 Apr 2008 19:03:44 -0700, Keith Thompson wrote:

>Harald van Dijk <tr*****@gmail.comwrites:

>>On Wed, 02 Apr 2008 15:39:32 -0400, lawrence.jones wrote:
Keith Thompson <ks***@mib.orgwrote:
Undefined behavior is illegal?

Well, the C Police aren't going to come and arrest you, but it's not
valid, which is what most people mean by "legal".

Most people don't consider the things illegal that the police turn a
blind eye towards. Undefined behaviour is what happens when your
program doesn't play by C's rules, but at the same time, C's rules tell
your compiler not to arrest you.

Actually, C's rules specifically *don't* tell the compiler not to arrest
you. Printing an error message and terminating, either at compile time
or at run time, is well within the (nonexistent) bounds of undefined
behavior.

I stand by my previous statement.

void f(){}
void g(){if(0)f(1);}

This has been declared a valid translation unit, and it may appear in
strictly conforming programs. A compiler is not allowed to reject it, as
long as the call to f is never executed, and in the general case, it's
not possible for the compiler to determine whether such a call would ever
be executed.

Right. Since the call f(1) never occurs, there's no undefined
behavior.

Perhaps I should have been clearer about the fact that UB can cause a
translation unit to be rejected by the compiler only if it can prove
that the UB will always happen, but I didn't think it was an important
point in context.

I'm not aware of any point on which we disagree.

--
Keith Thompson (The_Other_Keith) <ks***@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"