Re: Remove unordered containers iterators default initialization

[François Dumont <frs dot dumont at gmail dot com>]

On 11/26/2013 12:07 AM, Jonathan Wakely wrote:
> On 25 November 2013 21:02, François Dumont wrote:
> > Hi
> >
> >      Following N3644 discussion thread here is a patch proposal to remove
> > default zero-initialization of unordered containers iterator. I also took
> > the time to remove default zero-init of nodes _M_nxt pointer.
> >
> > 2013-11-25  François Dumont  <fdumont@gcc.gnu.org>
> >
> >      * include/bits/hashtable_policy.h (_Hash_node_base): Default
> >      default constructor.
> >      (_Node_iterator): Likewise.
> >      (_Node_const_iterator): Likewise.
> >      * include/bits/hashtable.h: Adapt.
> >
> > Tested under Linux x86_64.
> >
> > Ok to commit ?
> No, I still don't like the idea of leaving data intentionally
> uninitialized, so I don't like this part of the patch:
>
> Index: include/bits/hashtable_policy.h
> ===================================================================
> --- include/bits/hashtable_policy.h    (revision 205288)
> +++ include/bits/hashtable_policy.h    (working copy)
> @@ -230,7 +230,7 @@
>     {
>       _Hash_node_base* _M_nxt;
>
> -    _Hash_node_base() noexcept : _M_nxt() { }
> +    _Hash_node_base() = default;
>
>       _Hash_node_base(_Hash_node_base* __next) noexcept : _M_nxt(__next) { }
>     };
> @@ -306,6 +306,7 @@
>
>         __node_type*  _M_cur;
>
> +      _Node_iterator_base() = default;
>         _Node_iterator_base(__node_type* __p) noexcept
>         : _M_cur(__p) { }
>
>
> I don't think that will help diagnose errors.
>
> Dereferencing a null pointer will usually cause an immediate failure,
> dereferencing uninitialized pointers can have more subtle, harder to
> identify problems.
    Ok, I see, it is not a Standard conforming issue but we prefer to 
be user friendly to performance. I was hoping to make all library 
iterators trivially default constructible and allow some good 
optimizations (even if std::is_trivially_default_constructible is not 
yet implemented).

    Frankly, I think that detecting usage of uninitialized variables is 
a compiler job. Is it part of the compiler feature or on its TODOs list ?

François