# std::experimental::disjunction

From cppreference.com
< cpp‎ | experimental

Defined in header `<experimental/type_traits>` |  |
template< class... B > struct disjunction; |  | (library fundamentals TS v2)

Forms the logical disjunction of the type traits `B...`, effectively performing a logical or on the sequence of traits.

The specialization std::experimental::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`.

### Template parameters

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 > constexpr bool disjunction_v = disjunction<B...>::value; |  | (library fundamentals TS v2)

### Possible implementation

    template<class...> struct disjunction : std::false_type {};
    template<class B1> struct disjunction<B1> : B1 {};
    template<class B1, class... Bn>
    struct disjunction<B1, Bn...>
        : std::conditional_t<bool(B1::value), B1, disjunction<Bn...>>  {};

### Notes

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, disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.

### Example

| This section is incomplete Reason: no example

### See also

disjunction (C++17) | variadic logical OR metafunction (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/experimental/disjunction&oldid=154816"