Defect Report #463

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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


Harmonizing left-shift with C++14

It 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

6.5.7p4 should be modified to read:
The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 x 2E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 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.
C++ addressed this in C++14 with DR1457 with identical wording modifications.

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.

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