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# std::experimental::disjunction

From cppreference.com
< cpp‎ | experimental

| Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 2/2016, see `std::disjunction` (since C++17)

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

Forms the logical disjunction of the type traits `B...`.

The BaseCharacteristic of a specialization disjunction<B1, ..., BN> is the first `Bi` for which Bi::value != false, or if every Bi::value == false, the BaseCharacteristic is `BN`.

If sizeof...(B) == 0, the BaseCharacteristic is std::false_type.

Disjunction is short-circuiting: if there is a template type argument `Bi` with Bi::value != false, then instantiating disjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for `j > i`

### [edit] Template parameters

B... | - | every type must be usable as a base class and define member B::value that is convertible to bool

### [edit] Helper variable template

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

### [edit] 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<B1::value != false, B1, disjunction<Bn...>>  { };

### [edit] Notes

A specialization of `disjunction` does not necessarily have a BaseCharacteristic of either std::true_type or std::false_type: it simply inherits the base characteristic of the first B whose ::value, converted to bool, is true, or the base characteristic of 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.

### [edit] Example

| This section is incomplete Reason: no example

### [edit] See also

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

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