4.9 KiB
[meta.logical]
21 Metaprogramming library [meta]
21.3 Metaprogramming and type traits [type.traits]
21.3.10 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 1:
This is analogous to the short-circuiting behavior of the built-in operator &&.
â 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 2:
This means a specialization of conjunction does not necessarily inherit from either true_type or false_type.
â 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 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 3:
This is analogous to the short-circuiting behavior of the built-in operator ||.
â 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 4:
This means a specialization of disjunction does not necessarily inherit from either true_type or false_type.
â 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 is a Cpp17UnaryTypeTrait with a base characteristic of bool_constant<!bool(B::value)>.