On Thu, 2007-12-27 at 16:23 +0100, Tadeusz B. Kopec wrote:

> >
> > Output is,
> >
> > I am in constructor
> > I am in constructor
> > I am in copy constructor
> >
> > My expected output is,
> >
> > I am in constructor
> > I am in constructor
>
> The standard says that compiler is allowed to elide copy in return value
> but doesn't require it. Whether a specific compiler does this
> optimisation or not is implementation defined so off topic here.

I understood that the copy constructor must not be elided if it would
result in a different program. IE, any statistics gathering, reference
counting or custom allocators being used prevents elision (is that a
word?) except where the compiler can (and will) prove that the elided
version would give the same result in all cases.

--
Tristan Wibberley

Any opinion expressed is mine (or else I'm playing devils advocate for
the sake of a good argument). My employer had nothing to do with this
communication.

On 2007-12-27 23:06, Tristan Wibberley wrote:
> On Thu, 2007-12-27 at 16:23 +0100, Tadeusz B. Kopec wrote:
>
>> >
>> > Output is,
>> >
>> > I am in constructor
>> > I am in constructor
>> > I am in copy constructor
>> >
>> > My expected output is,
>> >
>> > I am in constructor
>> > I am in constructor
>>
>> The standard says that compiler is allowed to elide copy in return value
>> but doesn't require it. Whether a specific compiler does this
>> optimisation or not is implementation defined so off topic here.
>
> I understood that the copy constructor must not be elided if it would
> result in a different program. IE, any statistics gathering, reference
> counting or custom allocators being used prevents elision (is that a
> word?) except where the compiler can (and will) prove that the elided
> version would give the same result in all cases.

Actually no, the copy-constructor may always be elided, even if that
changes the effects of the program. That is why it is possible to
demonstrate the effects using print-outs in the copy constructor.
Because of this a copy-constructor should be designed to only copy an
object and nothing more, and you should never design your programs to
rely on the copy-constructor running.

--
Erik Wikström