Issue 1796: Is all-bits-zero for null characters a meaningful requirement? (original) (raw)

Title

Is all-bits-zero for null characters a meaningful requirement?

Status

cd4

Section

5.3.1 [lex.charset]

Submitter

Tony van Eerd

Created on 2013-10-02.00:00:00 last changed 107 months ago

Messages

Date: 2014-11-15.00:00:00

[Moved to DR at the November, 2014 meeting.]

Date: 2014-02-15.00:00:00

Proposed resolution (February, 2014):

Change 5.3.1 [lex.charset] paragraph 3 as follows:

The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character(respectively, null wide character), whose representation has all zero bits value is 0. For each basic execution character set...

Date: 2014-03-03.00:00:00

According to 5.3.1 [lex.charset] paragraph 3,

The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a_null character_ (respectively, null wide character), whose representation has all zero bits.

It is not clear that a portable program can examine the bits of the representation; instead, it would appear to be limited to examining the bits of the numbers corresponding to the value representation (6.9.2 [basic.fundamental] paragraph 1). It might be more appropriate to require that the null character value compare equal to 0 or '\0' rather than specifying the bit pattern of the representation.

There is a similar issue for the definition of shift, bitwiseand, and bitwise or operators: are those specifications constraints on the bit pattern of the representation or on the values resulting from the interpretation of those patterns as numbers?