BaseCharacteristic
, which is defined only for
UnaryTypeTraits
and BinaryTypeTraits
Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-10 Last modified: 2020-09-06
Priority: 2
View all other issues in [fund.ts.v2::meta.logical].
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Discussion:
Addresses: fund.ts.v2
The specification of conjunction
and disjunction
uses the term BaseCharacteristic
, which
is problematic in several ways:
That term is defined in 21.3.2 [meta.rqmts], but only for UnaryTypeTraits
and BinaryTypeTraits
.
conjunction
and disjunction
seem to be neither.
21.3.2 [meta.rqmts] also requires the BaseCharacteristic
for both UnaryTypeTraits
and
BinaryTypeTraits
to be a specialization of integral_constant
, which is inconsistent with the current design of
conjunction
and disjunction
.
The requirement in 21.3.2 [meta.rqmts] that "member names of the BaseCharacteristic
shall not be hidden
and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a
specialization of conjunction
or disjunction
derives may contain members called conjunction
or
disjunction
that will necessarily be hidden.
[2016-08 Chicago]
Ville provided wording for both 2567 and 2568.
[2016-08-07 Daniel provides wording borrowed from 2567]
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4600.
In 3.3.3 [fund.ts.v2::meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };-2- The class template
-3- Theconjunction
forms the logical conjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data membervalue
which is convertible tobool
, is not hidden, and is unambiguously available in the type.BaseCharacteristic of aspecializationconjunction<B1, …, BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listtrue_type, B1, ..., BN
for whichbool(Bi::value)
isfalse
, or- — if there is no such
Bi
, the last type in the list.-?- The member names of the base class, other than
is the first type[Note: This means a specialization of conjunction does not necessarilyB
in the listtrue_type, B1, …, BN
for whichB::value == false
, or ifevery B::value != false
the BaseCharacteristic is the last type in the list.have a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. — end note]conjunction
andoperator=
, shall not be hidden and shall be unambiguously available inconjunction
.
In 3.3.3 [fund.ts.v2::meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };-5- The class template
-6- Thedisjunction
forms the logical disjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data membervalue
which is convertible tobool
, is not hidden, and is unambiguously available in the type.BaseCharacteristic of aspecializationdisjunction<B1, …, BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listfalse_type, B1, ..., BN
for whichbool(Bi::value)
istrue
, or,- — if there is no such
Bi
, the last type in the list.-?- The member names of the base class, other than
is the first type[Note: This means a specialization of disjunction does not necessarilyB
in the listfalse_type, B1, …, BN
for whichB::value != false
, or ifevery B::value == false
the BaseCharacteristic is the last type in the list.have a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. — end note]disjunction
andoperator=
, shall not be hidden and shall be unambiguously available indisjunction
.
In 3.3.3 [fund.ts.v2::meta.logical] p8, edit as follows:
template<class B> struct negation :integral_constant<bool, !B::value>see below { };-8- The class template
negation
forms the logical negation of its template type argument. The typenegation<B>
is a UnaryTypeTrait with a BaseCharacteristic ofintegral_constant<bool, !bool(B::value)>
.