Re: Failing test when run as C++11

[Jonathan Wakely <jwakely at redhat dot com>]

On 09/05/14 12:20 +0100, Jonathan Wakely wrote:
> On 09/05/14 13:15 +0200, Daniel Krügler wrote:
> > 2014-05-09 12:53 GMT+02:00 Jonathan Wakely <jwakely@redhat.com>:
> > > 20_util/specialized_algorithms/uninitialized_copy/808590.cc fails if
> > > run in C++11 mode because is_nothrow_move_constructible<c> is false,
> > > which means vector::resize() copies the elements using non-const
> > > lvalue arguments, which selects the template constructor.
> > >
> > > (In C++03 it copies from *const* lvalue arguments, selecting the copy
> > > constructor.)
> > >
> > > We can either fix it by adding throw() to the copy constructor, or by
> > > adding a move cosntructor (with a comment explaining why):
> > >
> > > #if __cplusplus >= 201103L
> > >  c(c&&) = default;
> > > #endif
> > >
> > > I can't decide which I prefer, does anyone else have a preference?
> >
> > Do you know whether the test case was added to validate a bug fix and
> > if so, which one? I would like to ensure that the change still
> > reflects the original intention for adding it.
>
> See https://bugzilla.redhat.com/show_bug.cgi?id=808590
>
> That was filed against Red Hat's GCC 4.4 and at the time the upstream
> GCC already passed the test. Benjamin added the test to catch any
> regressions. Arguably, the C++11 behaviour (copying from non-const
> lvalue instead of const lvalue) can be considered a regression, but I
> think it's the correct behaviour.

To explain why I think it's correct...

In C++03 the vector::resize() copied objects via (the equivalent of)
allocator<c>::construct(pointer, const c&) which always calls the
copy constructor, c::c(const c&).

In C++11 it uses the allocator_traits equivalent of
allocator<c>::construct(pointer, Args&&...) which can call any
constructor, depending on the value category, cv-qualification and
type of the argument. When is_nothrow_move_constructible<c> is false
(as in the current test code) the argument is an lvalue, so we call
c::c<c>(c&) instead of c::c(const c&).

If is_nothrow_move_constructible<c> was true then the argument would
be forwarded as an rvalue, so the template constructor isn't viable
and we would call c::c(const c&), or c::c(c&&) if we add that.