23 General utilities library [utilities]

23.15 Metaprogramming and type traits [meta]

23.15.8 Logical operator traits [meta.logical]

1
#
This subclause describes type traits for applying logical operators to other type traits.
template<class... B> struct conjunction : see below { };
2
#
The class template conjunction forms the logical conjunction of its template type arguments.
3
#
For a specialization conjunction<B1, ..., BN>, if there is a template type argument Bi for which bool(Bi​::​value) is false, then instantiating conjunction<B1, ..., BN>​::​value does not require the instantiation of Bj​::​value for j > i.
[ Note
:
This is analogous to the short-circuiting behavior of the built-in operator &&.
— end note
 ]
4
#
Every template type argument for which Bi​::​value is instantiated shall be usable as a base class and shall have a member value which is convertible to bool, is not hidden, and is unambiguously available in the type.
5
#
The specialization conjunction<B1, ..., BN> has a public and unambiguous base that is either
  • (5.1)
    the first type Bi in the list true_­type, B1, ..., BN for which bool(Bi​::​value) is false, or
  • (5.2)
    if there is no such Bi, the last type in the list.
[ Note
:
This means a specialization of conjunction does not necessarily inherit from either true_­type or false_­type.
— end note
 ]
6
#
The member names of the base class, other than conjunction and operator=, shall not be hidden and shall be unambiguously available in conjunction.
template<class... B> struct disjunction : see below { };
7
#
The class template disjunction forms the logical disjunction of its template type arguments.
8
#
For a specialization disjunction<B1, ..., BN>, if there is a template type argument Bi for which bool(Bi​::​value) is true, then instantiating disjunction<B1, ..., BN>​::​value does not require the instantiation of Bj​::​value for j > i.
[ Note
:
This is analogous to the short-circuiting behavior of the built-in operator ||.
— end note
 ]
9
#
Every template type argument for which Bi​::​value is instantiated shall be usable as a base class and shall have a member value which is convertible to bool, is not hidden, and is unambiguously available in the type.
10
#
The specialization disjunction<B1, ..., BN> has a public and unambiguous base that is either
  • (10.1)
    the first type Bi in the list false_­type, B1, ..., BN for which bool(Bi​::​value) is true, or
  • (10.2)
    if there is no such Bi, the last type in the list.
[ Note
:
This means a specialization of disjunction does not necessarily inherit from either true_­type or false_­type.
— end note
 ]
11
#
The member names of the base class, other than disjunction and operator=, shall not be hidden and shall be unambiguously available in disjunction.
template<class B> struct negation : see below { };
12
#
The class template negation forms the logical negation of its template type argument.
The type negation<B> is a UnaryTypeTrait with a base characteristic of bool_­constant<!bool(B​::​value)>.