std::disjunction (3) - Linux Man Pages
Defined in header <type_traits>
template<class... B> (1) (since C++17)
Forms the logical_disjunction of the type traits B..., effectively performing a logical OR on the sequence of traits.
The specialization std::disjunction<B1, ..., BN> has a public and unambiguous base that is
* if sizeof...(B) == 0, std::false_type; otherwise
* the first type Bi in B1, ..., BN for which bool(Bi::value) == true, or BN if there is no such type.
The member names of the base class, other than disjunction and operator=, are not hidden and are unambiguously available in disjunction.
Disjunction is short-circuiting: if there is a template type argument Bi with bool(Bi::value) != false, then instantiating disjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for j > i
B... - every template argument Bi for which Bi::value is instantiated must be usable as a base class and define member value that is convertible to bool
Helper variable template
template<class... B> (since C++17)
inline constexpr bool disjunction_v = disjunction<B...>::value;
A specialization of disjunction does not necessarily inherit from of either std::true_type or std::false_type: it simply inherits from the first B whose ::value, explicitly converted to bool, is true, or from the very last B when all of them convert to false. For example, std::disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.
The short-circuit instantiation differentiates disjunction from fold expressions: a fold expression like (... || Bs::value) instantiates every B in Bs, while std::disjunction_v<Bs...> stops instantiation once the value can be determined. This is particularly useful if the later type is expensive to instantiate or can cause a hard error when instantiated with the wrong type.
// Run this code