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Re: std::forward_list optim for always equal allocator

On 11/09/17 07:44 +0200, Daniel Krügler wrote:
2017-09-11 7:12 GMT+02:00 François Dumont <>:
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

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

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

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