🚀 go-pugleaf

damian@molly.cs.monash.edu.au (Damian Conway) writes:

>kanze <kanze@lts.sel.alcatel.de> writes:

>>In article <45sl3a$coc@engnews2.Eng.Sun.COM> clamage@Eng.sun.com
>>(Steve Clamage) writes:

>>|> damian@molly.cs.monash.edu.au (Damian Conway) writes:

>>|> >Then is it possible to set up a class X wherein:

>>|> >	X x(1);
>>|> >and
>>|> >	X x = 1;

>>|> >behave differently?

>>|> Yes.  Silly example: Write a message to the terminal from the copy
>>|> constructor. If the copy is optimized away, you won't see the message.

>>I don't think that is what he was looking for.  As far as I know,
>>there is no way of *ensuring* that the above will both be legal, but
>>have garanteed different run-time semantics.

>Yes, that was what I was asking.

>>It is trival to make the first legal, and the second illegal, which, I
>>suppose is a way of making them behave differently at compile time.

>:-)

>BTW: What was the rationale for making something as fundamental as copy
>     initialization non-portable (in the sense that it has no guaranteed
>     semantics)?


While it is not easy to write an example in which

X x(1); and X x = 1;  are guaranteed to behave differently, it is quite
easy to produce a case in which X x(a); and X x=a; behave differently,
where a is a class object, of a class that has a conversion operator to
X&, and another higher priority conversion to something accepted by a
constructor of class X. then:

X x(a) uses higher priority conversion by overloading rules

X x=a uses X(a.operator X&)

At least this is the way I think it works.
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