Re: PR libstdc++/68222 Hide safe iterator operators

[Jonathan Wakely <jwakely at redhat dot com>]

On 02/08/18 22:16 +0200, François Dumont wrote:
> Hi
>
>     Here is a patch to avoid definition of invalid operators on the 
> Debug mode safe iterator type depending on its category.
>
>     Even if it is limited to the Debug mode code I would like to have 
> a feedback before committing. Especially on the following points:
>
> - _Safe_tagged_iterator: Is the name ok ?

Hmm, maybe "strict" instead of tagged?

But do we need a new name? Can we just change _Safe_iterator instead
of adding a new type?

Where is _Safe_iterator still used? Just local iterators in unordered
containers?  Is it OK to remove most of the functions that used to
support it? (__niter_base etc).

Could we add a new type for the local iterators, and just change
_Safe_iterator directly so it doesn't expose unsupported operations?

That would make the patch *much* smaller, as you wouldn't need to
change all the uses of _Safe_iterator.


Another approach would be to use mixins to expose the operations:

template<typename _Iter, typename _Cat>
struct _Safe_iterator_mixin<_Iter, _Cat>
{
 typename iterator_traits<_Iter>::reference
 operator*()
 { return static_cast<_Iter*>(this)->_M_deref(); }
};

template<typename _Iter, typename _Cat>
struct _Safe_iterator_mixin<_Iter, forward_iterator_tag>
{
 _Iter& operator++()
 { return static_cast<_Iter*>(this)->_M_preinc(); }
 _Iter operator++(int)
 { return static_cast<_Iter*>(this)->_M_postinc(); }
};

template<typename _Iter, typename _Cat>
struct _Safe_iterator_mixin<_Iter, bidirectional_iterator_tag>
{
 _Iter& operator--()
 { return static_cast<_Iter*>(this)->_M_predec(); }
 _Iter operator--(int)
 { return static_cast<_Iter*>(this)->_M_postdec(); }
};

etc.

then in _Safe_iterator rename the operator functions, so operator*
becomes _M_deref, operator++ becomes _M_preinc etc. and then derive
from _Safe_iterator_mixin which declares the operators.


> - Inheritance between the different _Safe_tagged_iterator 
> instantiations. I am already working on making the operators friends 
> as we discuss in another thread so I might review this design at this 
> moment.
>
> - Are concept checks I added to testsuite_containers.h citerator ok ?

Yes, they look useful.

>     This patch also does some cleanup on Debug functions. 
> __check_dereferenceable was not used (anymore maybe) so I removed it. 

It would have made the patch a bit more manageable to keep that as a
separate patch, but nevermind.

> diff --git a/libstdc++-v3/include/debug/deque b/libstdc++-v3/include/debug/deque
> index 93b82cf..213e23b 100644
> --- a/libstdc++-v3/include/debug/deque
> +++ b/libstdc++-v3/include/debug/deque
> @@ -57,12 +57,14 @@ namespace __debug
>       typedef __gnu_debug::_Equal_to<_Base_const_iterator> _Equal;
>
>     public:
> +      typedef _Base					normal_type;

normal_type is not a reserved name, so can't be used.


> diff --git a/libstdc++-v3/include/debug/stl_iterator.h b/libstdc++-v3/include/debug/stl_iterator.h
> index f20b000..fd20160 100644
> --- a/libstdc++-v3/include/debug/stl_iterator.h
> +++ b/libstdc++-v3/include/debug/stl_iterator.h
> @@ -75,12 +76,6 @@ namespace __gnu_debug
> #else
>   template<typename _Iterator>
>     inline auto
> -    __base(const std::reverse_iterator<_Iterator>& __it)
> -    -> decltype(std::__make_reverse_iterator(__base(__it.base())))
> -    { return std::__make_reverse_iterator(__base(__it.base())); }

This removal doesn't seem to be mentioned in the ChangeLog:

> 	* include/debug/stl_iterator.h
> 	(__is_safe_random_iterator<std::reverse_iterator<>>): Remove.
> 	(__base(const std::reverse_iterator<_Safe_tagged_iterator<_It, _Sq,
> 	std::random_access_iterator_tag>)): New overload.