Re: N3644 Comparing value-initialized iterators

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

On 11/15/2013 07:26 PM, Jonathan Wakely wrote:
> http://www.open-std.org/JTC1/sc22/WG21/docs/papers/2013/n3644.pdf
> makes it OK to compare two value-initialized iterators, and is one of
> the few C++14 library features not implemented on trunk.
>
> I haven't checked, but my assumption is that the only change needed to
> the library is to allow examples like this in Debug Mode:
>
>    std::vector<int>::iterator i{}; // value init
>    i == i;
>
> Currently this fails because the iterator is singular, but we need to
> allow it, *without* allowing this:
>
>    std::vector<int>::iterator i;  // default init
>    i == i;
>
> I'm not sure how to do this. I thought about trying to play games with
> uninitialized members that would be zero-init'd by value-init and left
> uninit'd by default-init, but I gave up.

    Why ? It sounds like the right approach to me too.

    However there is a small issue to this approach. Uninitialized 
memory might look like zero-init one so we will surely miss some invalid 
usages. But it is better than false positive so we will just have to 
live with it as I don't think we can do otherwise.

    The other reason why it won't work great for the moment is that 
there are some useless initialization. To begin with the 
_Safe_iterator_base which is zero initializing _M_version, _M_prior and 
_M_next per default. Some normal iterators are also doing zero 
initialization for nothing, at least the unordered container iterators 
are doing so. By removing those default zero-init we would enhance our 
probability of not missing any invalid usage.

    I will spend some time on it if you don't mind.

François