[LWG #280] std::reverse_iterator missing template assignment operator

• Key: STDCXX-689
• Type: Bug
• Status: Open
• Priority: Trivial
• Assignee: Unassigned
• Reporter: Martin Sebor
• Component/s: 24. Iterators
• Affects Version/s: 4.1.2, 4.1.3, 4.1.4, 4.2.0
• Fix Version/s: 4.3.0

Description

LWG Issue 280 added a template assignment operator to reverse_iterator:
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#280

It doesn't look like we ever added the function.

I'm having trouble coming up with a test case that would reveal its absence so this is just a placeholder to remind us to look into this in more detail to make sure the function really is needed (if not, there's no need to complicate the interface of the template).

Comments

[Travis Vitek]

It seems that changing this would introduce a source incompatibility.

  int a [10] = {0};

  std::reverse_iterator<      int*>  rvi (&a [0]);
  std::reverse_iterator<const int*> crvi (&a [0]);
  
  // this is std::operator== (const std::reverse_iterator<int*>&, 
  //                          const std::reverse_iterator<const int*>&)
   rvi == crvi; // also for !=, <, <=, >, >= and -
  crvi == rvi;

The above code would compile with new versions of the library, but not with older versions of the library. Is this change suitable for 4.2.1? It seems that it should not be.

As an aside, I'm with John Potter on wg issues 179 and 280... I think the issue is in support of bad code. Compares should be between two iterators of the same type.

[Martin Sebor]

I suspect I hadn't realized issue 280 was about more than the assignment operator. Or I hadn't intended to include more than the assignment operator portion of issue 280 in this issue. Either way, I now tend to think they might as well be dealt with together. With that in mind, I agree that changing the equality operators to tale two template parameters is strictly speaking an incompatible change that would be safest to deferred until 4.3.

[Travis Vitek]

Added patch.

[Travis Vitek]

This change would not be forward source compatible. Deffered to 4.3.