19 General utilities library [utilities]

19.15 Metaprogramming and type traits [meta]

19.15.8 Logical operator traits [meta.logical]

This subclause describes type traits for applying logical operators to other type traits.
template<class... B> struct conjunction : see below { };
The class template conjunction forms the logical conjunction of its template type arguments.
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
]
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.
The specialization conjunction<B1, ..., BN> has a public and unambiguous base that is either
  • the first type Bi in the list true_­type, B1, ..., BN for which bool(Bi::value) is false, or
  • 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
]
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 { };
The class template disjunction forms the logical disjunction of its template type arguments.
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
]
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.
The specialization disjunction<B1, ..., BN> has a public and unambiguous base that is either
  • the first type Bi in the list false_­type, B1, ..., BN for which bool(Bi::value) is true, or
  • 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
]
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 { };
The class template negation forms the logical negation of its template type argument.
The type negation<B> is a Cpp17UnaryTypeTrait with a base characteristic of bool_­constant<!bool(B::​value)>.