jason@cygnus.com (Jason Merrill) writes:

>>>>>> Robert Schmidt <robert@idt.unit.no> writes:
>
>> No, compilers need not report this as an error.  The lifetime of a
>> temporary has been the cause of much discussion -
>
>Lifetime of temporaries isn't the issue here; the question is whether or
>not you can bind a non-const reference to a temporary, and the answer is
>currently no, since the temporary is not an lvalue.

OK, so answers my first question - the program is illegal.
(Someone else noted that there is at least one compiler which rejects it.)

Now what about the second - what happens if you replace
 X x(foo());
with
 X x = foo();
?
Now the compiler is allowed to eliminate the temporary - does that
mean that it doesn't have to report an error?  Or must it do
full error-checking *before* attempting to eliminate temporaries?

--
Fergus Henderson - fjh@munta.cs.mu.oz.au

>>>>> Fergus Henderson <fjh@munta.cs.mu.OZ.AU> writes:

> Now what about the second - what happens if you replace
>  X x(foo());
> with
>  X x = foo();
> ?
> Now the compiler is allowed to eliminate the temporary - does that
> mean that it doesn't have to report an error?  Or must it do
> full error-checking *before* attempting to eliminate temporaries?

I would argue that since elimination of temporaries falls under "as-if", it
cannot be done if doing so would change the semantics.

Reading the standard like this leads me to conclude that you cannot
initialize an object with a copy constructor that takes a non-const
reference from a function returning by value.  Or, for that matter, from an
explicit constructor call, as in the common example

complex c = complex (1, 2);

since an explicit constructor call returns a temporary.

Jason

fjh@munta.cs.mu.OZ.AU (Fergus Henderson) writes:

>Is the following program strictly conforming to the ARM and/or latest
>working paper?
>
> struct X {
>  X(X&) {}

Oops.  Of course I also want:

  X() {}

> };
> X foo() { X x; return x; }
>
> int main() {
>  X x(foo());
> }

--
Fergus Henderson - fjh@munta.cs.mu.oz.au