Re: std::forward_list optim for always equal allocator

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

On 08/09/2017 18:19, Jonathan Wakely wrote:
> On 28/08/17 21:09 +0200, François Dumont wrote:
> > Hi
> >
> >    Any news for this patch ?
> >
> >    It does remove a constructor:
> > -        _Fwd_list_impl(const _Node_alloc_type& __a)
> > -        : _Node_alloc_type(__a), _M_head()
> >
> >     It was already unused before the patch. Do you think it has ever 
> > been used and so do I need to restore it ?
> >
> >    I eventually restore the _M_head() in _Fwd_list_impl constructors 
> > cause IMO it is the inline init of _M_next in _Fwd_list_node_base 
> > which should be removed. But I remember Jonathan that you didn't want 
> > to do so because gcc was not good enough in detecting usage of 
> > uninitialized variables, is it still true ?
>
> Why should it be removed?
When user declare a container iterator like that:

std::forward_list<int>::iterator it;

There is no reason to initialize it with a null node pointer. It is just 
an uninitialized iterator which is invalid to use except to initialize it.

I once proposed to do the same simplification for the unordered 
containers _Hash_node_base. But you said that detection of the usage of 
uninitialized variable is not good enough in gcc to leave variables 
uninitialized this way.