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2519. Iterator operator-= has gratuitous undefined behaviour

Section: 25.3.5.7 [random.access.iterators] Status: C++17 Submitter: Hubert Tong Opened: 2015-07-15 Last modified: 2017-07-30

Priority: 2

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Discussion:

In subclause 25.3.5.7 [random.access.iterators], Table 110, the operational semantics for the expression "r -= n" are defined as

return r += -n;

Given a difference_type of a type int with range [-32768, 32767], if the value of n is -32768, then the evaluation of -n causes undefined behaviour (Clause 5 [expr] paragraph 4).

The operational semantics may be changed such that the undefined behaviour is avoided.

Suggested wording:

Replace the operational semantics for "r -= n" with:

{ 
  difference_type m = n;
  if (m >= 0)
    while (m--)
      --r;
  else
    while (m++)
      ++r;
  return r; 
}

Jonathan Wakely:

I'm now convinced we don't want to change the definition of -= and instead we should explicitly state the (currently implicit) precondition that n != numeric_limits<difference_type>::min().

[2016-08, Chicago]

Monday PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Change Table 110 "Random access iterator requirements (in addition to bidirectional iterator)" as indicated:

   Table 110 — Random access iterator requirements (in addition to bidirectional iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   …
   r -= n	X&	return r += -n;	pre: the absolute value of n is in the range of representable values of difference_type.
   …