std::forward_list optim for always equal allocator

[Jonathan Wakely]

On 11/09/17 07:44 +0200, Daniel Krügler wrote:
>2017-09-11 7:12 GMT+02:00 François Dumont <frs.dumont@gmail.com>:
>> 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.
>
>While that is correct, for every forward iterator (and
>std::forward_list<int>::iterator meets these requirements), it is also
>required that a value-initialized iterator can be compared against
>other initialized iterators, so this reduces the amount of freedom to
>define a default constructor for such iterators even when used to
>default-initialize. This is not meant as a showstopper argument, since
>I have not fully understood of what you are planning, but just a
>reminder.

Right, which means that 

std::forward_list<int>::iterator it = {};

must initialize the node pointer to nullptr. If we remove the
initialization of _Fwd_list_iterator<T>::_M_node from the default
constructor then it would be left uninitialized.

But I'm confused, François was talking about removing the
initialization of _Fwd_list_node_base::_M_next, what has that got to
do with forward_list<T>::iterator? Thee is no node-base in the
iterator.

So I'm still wondering why the initialization of _M_next should be
removed.