Submitter: Aaron Ballman
Submission Date: 2014-04-02
Source: WG14
Reference Document: N1584
Version: 1.0
Date: April 2014
Subject: Left-shifting into the sign bit
Summary
Harmonizing left-shift with C++14It is not uncommon to see code such as:
signed someint_t min_value = 1 << (CHAR_BIT * sizeof(someint_t));
However, left-shifting a one bit into the sign bit is undefined behavior, despite the fact that the majority of (twos-complement) architectures handle it properly.
Suggested Technical Corrigendum
The result ofC++ addressed this in C++14 with DR1457 with identical wording modifications.E1 << E2
isE1
left-shiftedE2
bit positions; vacated bits are filled with zeros. IfE1
has an unsigned type, the value of the result isE1 x 2E2
, reduced modulo one more than the maximum value representable in the result type. IfE1
has a signed type and nonnegative value, andE1 x 2E2
is representable in the corresponding unsigned type of the result type, then that value, converted to the result type, is the resulting value; otherwise, the behavior is undefined.
Apr 2014 meeting
Proposed Committee Response
This is not a defect.The committee will track this and consider it for the next revision of the standard.