What's the rationale for the rule of comma operator?

[Lighter]

In 5.3.3.4 of the standard, the standard provides that "The lvalue-to-
rvalue(4.1), array-to-pointer(4.2),and function-to-pointer(4.3)
standard conversions are not applied to the operand of sizeof."

I think this rule is easy to understand. Because I can find the
contexts of applying the rule as follows.

(1)
int* p = 0;
int b1 = sizeof(*p); // OK, b1 = 4, *p would not be evaluated.

(2)
int array[2];
int b2 = sizeof(array); // OK, b2 = 8, array will not be automatically
converted to a pointer to int

(3)
int b3 = sizeof(strlen); // error, strlen will not be automatically
converted to a function pointer.

So far, everything is understandable.
==========================================

However, In 5.18 of the standard, I find a similar rule on comma
operator: "The lvalue-to-rvalue(4.1), array-to-pointer(4.2),and
function-to-pointer(4.3) standard conversions are not applied to the
left expression."

I tried my best to guess the reason of the latter rule, but failed.
Even, I could not find an example to examine the rule.

As far as I know, C99 didn't provide the rule. My confusion lies in
the reason for making such a rule. Could you give me an example
illustrating that some conversions are applied on the left expression
(or the left operand)? I could not find such an example. If there were
no such an example. Why did the standard provide such a rule?

I cannot even see how one would check whether the conversion happens
or not. Since one couldn't check whether the conversion happens or
not, WHY did the C++ standard explicitly provide such a rule which
doesn't exist in the C language standard? What confused me is just the
question.

Any help will be highly appreciated. Thanks in advance.

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[Hyman Rosen]

If the left operand of a comma operator had the
lvalue-to-rvalue conversion applied, the following
program would become illegal

extern struct A a;
int main() { return a, 0; }

because the conversion may not be applied to an
object of incomplete type.

[Andrew Koenig]

"Lighter" <cqu...@gmail.com> wrote in message
news:1182447043.8...@x35g2000prf.googlegroups.com...

> As far as I know, C99 didn't provide the rule. My confusion lies in
> the reason for making such a rule. Could you give me an example
> illustrating that some conversions are applied on the left expression
> (or the left operand)? I could not find such an example. If there were
> no such an example. Why did the standard provide such a rule?

Part of the reason for the rule is to ensure that when someone writes code
such as

(std::cout << foo), bar();

the implementation is not permitted to reject it even though std::cout is
not copyable.

[James Kanze]

On Jun 22, 11:31 am, a...@acm.org ("Andrew Koenig") wrote:
> "Lighter" <cqu...@gmail.com> wrote in message

> news:1182447043.8...@x35g2000prf.googlegroups.com...

> > As far as I know, C99 didn't provide the rule. My confusion lies in
> > the reason for making such a rule. Could you give me an example
> > illustrating that some conversions are applied on the left expression
> > (or the left operand)? I could not find such an example. If there were
> > no such an example. Why did the standard provide such a rule?

> Part of the reason for the rule is to ensure that when someone writes code
> such as

> (std::cout << foo), bar();

> the implementation is not permitted to reject it even though std::cout is
> not copyable.

What's the relationship between copiable and the rest? Copiable
has nothing to do with array to pointer or function to pointer
conversions, and for a class type, it doesn't involve an
lvalue to rvalue conversion.

Does lvalue to rvalue conversion apply to class types, anyway?
I can't think of a case where it would. You copy a class by
calling the copy constructor, which means binding to a
reference, and not lvalue to rvalue conversion. In fact, all
legal operations on a class type are "lvalue-neutral", I think.
The only place I see where lvalue-ness plays a role is in
determining whether you can bind to a non-const reference or
not.

(But I've probably overlooked something fundamental.)

--
James Kanze (GABI Software, from CAI) email:james...@gmail.com
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