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From: Leigh Johnston on 17 Feb 2010 05:25


"Goran" <goran.pusic(a)gmail.com> wrote in message
news:9af8d152-6452-46f9-8f6c-488212df22df(a)b2g2000yqi.googlegroups.com...

> On Feb 16, 11:35 pm, "Leigh Johnston" <le...(a)i42.co.uk> wrote:
>
>> Hi,
>>
>> I am slightly worried by the example in 3.8.7, specifically the following
>> line:
>>
>> new (this) C(other); // new object of type C created
>>
>> Although not the case in this example if C's copy constructor could throw
>> an
>> exception a destructor could be erroneously called (destruction of c1 in
>> the
>> example during stack unwinding).
>>
>
> What you say is correct. What's done there is __not__ what one should
> do as a normal course of action. One __can__ do that when C(const C&)
> does not throw. But even then, that line you've shown most often
> likely should be preceded by this->~C(). That is, "this" should likely
> be "destroyed" as a normal course of action (although one could
> imagine a situation where even this is not needed).
>

The example I am referring to does have "this->~C()" before the placement
new yes, which is part of the problem: object is now destroyed and can be
erroneously destroyed a second time if the copy constructor throws as the
object is (was) on the stack.


> Correct way is to have assignment operator and do simply
>
> *this = other;
>
> Wrong reasons why people do this sort of thing:
> * they don't know any better
> * laziness (prick couldn't be bothered to write assignment operator)
> * premature optimization ("I know, I'll recreate object in-place,
> that'll be faster!")
>
> Goran.
>

Yes I know this which is the reason why I posted with my worries about this
example as it is not exception safe when considered as a general "pattern" -
not good quality for something in a standard document I reckon.


> P.S. What's 3.8.7. and N3000, BTW?
>
>
Section 3.8.7 of the draft C++0x standard (v N3000).

/Leigh



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From: =3D?ISO-8859-1?Q?Daniel_Kr=3DFCgler?=3D on 17 Feb 2010 05:24
On 17 Feb., 18:17, Goran <goran.pu...(a)gmail.com> wrote:
> P.S. What's 3.8.7. and N3000, BTW?

N3000 is a public accessible document of the current working
/draft/ of the upcoming C++0x standard. Just visit

http://www.open-std.org/jtc1/sc22/wg21/

search for the "papers" link at the top, then select the most
recent 2009 block, unit "2009-11 post-Santa Cruz mailing".
In the list of papers you'll find

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf

3.8.7 is a wrong quote. It is supposed to mean 3.8/7 or
[basic.life]/p. 7

HTH & Greetings from Bremen,

Daniel Kr=FCgler


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From: Goran on 18 Feb 2010 00:19
On Feb 17, 11:24 pm, "Leigh Johnston" <le...(a)i42.co.uk> wrote:
> > Correct way is to have assignment operator and do simply
>
> > *this = other;
>
> I disagree, this results in infinite recursion (or finite with stack fault
> given a finite stack size)!

Eh? No, it doesn't. Why would it? What exactly did you do to get
recursion (code snippet)?

Goran.


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From: Leigh Johnston on 19 Feb 2010 03:47


"Goran" <goran.pusic(a)gmail.com> wrote in message
news:4b9a51fe-4334-45e1-86d5-c0da965dc668(a)q29g2000yqn.googlegroups.com...
> On Feb 17, 11:24 pm, "Leigh Johnston" <le...(a)i42.co.uk> wrote:
>> > Correct way is to have assignment operator and do simply
>>
>> > *this = other;
>>
>> I disagree, this results in infinite recursion (or finite with stack
>> fault
>> given a finite stack size)!
>
> Eh? No, it doesn't. Why would it? What exactly did you do to get
> recursion (code snippet)?
>
> Goran.
>

foo& foo::operator=(const foo& other)
{
*this = other; // this is recursive, i.e. it calls itself again.
return *this;
}

/Leigh


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From: Goran on 22 Feb 2010 02:04
On Feb 19, 9:47 pm, "Leigh Johnston" <le...(a)i42.co.uk> wrote:
> > Eh? No, it doesn't. Why would it? What exactly did you do to get
> > recursion (code snippet)?
>
> > Goran.
>
> foo& foo::operator=(const foo& other)
> {
> *this = other; // this is recursive, i.e. it calls itself again.
> return *this;
>
> }

Ah... OK, here's where misunderstanding arose: when I wrote that one
should have operator= and simply do *this = other, I did not mean (nor
say) that one should do *this = other inside operator=. I merely
wanted to say that one should not use... ahem... "fake copy
construction with placement new", but simply operator=. But it seems
that in your example they did it __inside__ operator= (did they? I
didn't look). If so, whoops, yes, you're right, it is indeed an
infinite recursion.

BTW, requiring that copy constructor must not throw is not good
enough. For example, it is sufficient to have member of C that has a
non-trivial destructor (e.g. frees a resource). That easily leaks
memory (said member is constructed two times, but destructed once, so
a resource is leaked once).

Goran.


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