[meta.logical] (original) (raw)
21 Metaprogramming library [meta]
21.3 Metaprogramming and type traits [type.traits]
21.3.9 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 conjunctionforms the logical conjunction of its template type arguments.
For a specialization conjunction<, …, >, if there is a template type argument for which bool(::value) is false, then instantiating conjunction<, …, >::valuedoes not require the instantiation of ::value for .
[Note 1:
This is analogous to the short-circuiting behavior of the built-in operator &&.
— _end note_]
Every template type argument for which ::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<, …, >has a public and unambiguous base that is either
- the first type in the list true_type, , …, for which bool(::value) is false, or
- if there is no such , the last type in the list.
[Note 2:
This means a specialization of conjunctiondoes not necessarily inherit from either true_type or false_type.
— _end note_]
The member names of the base class, other than conjunction andoperator=, shall not be hidden and shall be unambiguously available in conjunction.
template<class... B> struct disjunction : _see below_ { };
The class template disjunctionforms the logical disjunction of its template type arguments.
For a specialization disjunction<, …, >, if there is a template type argument for which bool(::value) is true, then instantiating disjunction<, …, >::valuedoes not require the instantiation of ::value for .
[Note 3:
This is analogous to the short-circuiting behavior of the built-in operator ||.
— _end note_]
Every template type argument for which ::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<, …, >has a public and unambiguous base that is either
- the first type in the list false_type, , …, for which bool(::value) is true, or
- if there is no such , the last type in the list.
[Note 4:
This means a specialization of disjunctiondoes 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 negationforms 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)>.