[Bug libstdc++/106175] New: std::prev and next should check for equality with std::begin

[Bug libstdc++/106175] New: std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

            Bug ID: 106175
           Summary: std::prev and next should check for equality with
                    std::begin
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: deco33000 at yandex dot com
  Target Milestone: ---

Hi,

To get better defined algorithms, we should not have a segfault in that case:

int main() 
{
    std::vector<int> v;

    // if (v.empty()) {return;}; <- should not need that line

    for (auto i = begin(v); i != prev(end(v)); i += 1){

        std::cout << *i << std::endl;

    } 

    return 0;

}

std::prev should always check for equality with begin() or empty() for its
argument.
If equal, do nothing to the iterator. The loop won't be entered.

Same for std::next, and by extension std::advance.

if we need to do specific actions in case the vector is empty, that is an
opt-in decision.

There may be some good counter arguments but I don't see them..

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
How do you get the begin iterator from another iterator? That's impossible in
general. The std::prev function has no access to the vector, it can't call
begin on it.

For the debug mode, we already abort when you use prev on the begin iterator:


In function:
    gnu_debug::_Safe_iterator<_Iterator, _Sequence, 
    std::random_access_iterator_tag>& gnu_debug::_Safe_iterator<_Iterator, 
    _Sequence, std::random_access_iterator_tag>::operator+=(difference_type) 
    [with _Iterator = gnu_cxx::normal_iterator<int*, std::vector<int, 
    std::allocator<int> > >; _Sequence = std::debug::vector<int>; 
    difference_type = long int]

Error: attempt to advance a past-the-end iterator -1 steps, which falls 
outside its valid range.

Objects involved in the operation:
    iterator @ 0x7fff987ee7a0 {
      type = gnu_cxx::normal_iterator<int*, std::vector<int,
std::allocator<int> > > (mutable iterator);
      state = past-the-end;
      references sequence with type 'std::debug::vector<int,
std::allocator<int> >' @ 0x7fff987ee730
    }

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

--- Comment #2 from KL <deco33000 at yandex dot com> ---
During the parsing stage, the compiler has this information.

It knows that we want the end of v.
So it can inject a comparison into the resulting code.

A segfault here is not acceptable for a language like C++. There is no trap for
the compiler here.

That comparison is cheap.

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

--- Comment #3 from KL <deco33000 at yandex dot com> ---
Please note, that comparison should only occur when you use next, prev or
advance.
It is useless for normal operations using end() directly. So no cost here.

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

KL <deco33000 at yandex dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|WORKSFORME                  |---
             Status|RESOLVED                    |UNCONFIRMED

--- Comment #4 from KL <deco33000 at yandex dot com> ---
it is not resolved as is

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
> A segfault here is not acceptable for a language like C++.

Well no. This is how the language is defined. It is undefined which means you
should have a check before.

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

KL <deco33000 at yandex dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|WORKSFORME                  |---
             Status|RESOLVED                    |UNCONFIRMED

--- Comment #6 from KL <deco33000 at yandex dot com> ---
And this is not a normal situation for a language that aims to be safer.

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to KL from comment #6)
> And this is not a normal situation for a language that aims to be safer.

GCC's debug mode for libstdc++ detects it. If you don't want to use that while
debugging your program, then you should raise this up to the standards
committee ...

[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

--- Comment #8 from KL <deco33000 at yandex dot com> ---
Sending a message to the commitee is like sending a message to void.

Issue resolved.