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### 1450. Contradiction in `regex_constants`

Section: 32.4.3 [re.matchflag] Status: C++14 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: 3

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Discussion:

Addresses GB-127

The Bitmask Type requirements in 16.3.3.3.4 [bitmask.types] p.3 say that all elements on a bitmask type have distinct values, but 32.4.3 [re.matchflag] defines `regex_constants::match_default` and `regex_constants::format_default` as elements of the bitmask type `regex_constants::match_flag_type`, both with value 0. This is a contradiction.

[ Resolution proposed by ballot comment: ]

> One of the bitmask elements should be removed from the declaration and should be defined separately, in the same manner as `ios_base::adjustfield`, `ios_base::basefield` and `ios_base::floatfield` are defined by 99 [ios::fmtflags] p.2 and Table 120. These are constants of a bitmask type, but are not distinct elements, they have more than one value set in the bitmask. `regex_constants::format_default` should be specified as a constant with the same value as `regex_constants::match_default`.

[ 2010-10-31 Daniel comments: ]

Strictly speaking, a bitmask type cannot have any element of value 0 at all, because any such value would contradict the requirement expressed in 16.3.3.3.4 [bitmask.types] p. 3:

> for any pair Ci and Cj, Ci & Ci is nonzero

So, actually both `regex_constants::match_default` and `regex_constants::format_default` are only constants of the type `regex_constants::match_flag_type`, and no bitmask elements.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

The proposed resolution is written against N3126 and considered as a further improvement of the fixes suggested by n3110.

> Add the following sentence to 32.4.3 [re.matchflag] paragraph 1:

> > 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set. Type `regex_constants::match_flag_type` also defines the constants `regex_constants::match_default` and `regex_constants::format_default`.

[ 2011 Bloomington ]

It appears the key problem is the phrasing of the bitmask requirements. Jeremiah supplies updated wording.

Pete Becker has also provided an alternative resolution.

Ammend 16.3.3.3.4 [bitmask.types]:

Change the list of values for "enum bit mask" in p2 from

`V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ...`.

to

`V0 = 0, V1 = 1 << 0, V2 = 1 << 1, V3 = 1 << 2, ...`.

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such non-zero elements have distinct values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero.

Change bullet 3 of paragraph 4:

The A non-zero value Y is set in the object X if the expression X & Y is nonzero.

[2014-02-13 Issaquah:]

Proposed resolution:

Ammend 16.3.3.3.4 [bitmask.types] p3:

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero. Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.

Add the following sentence to 32.4.3 [re.matchflag] paragraph 1:

> 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). The constants of that type, except for `match_default` and `format_default`, are bitmask elements. The `match_default` and `format_default` constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set.