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# std::disjunction

From cppreference.com
< cpp‎ | types

Defined in header `<type_traits>` |  |
template<class... B> struct disjunction; | (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`

### [edit] 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

### [edit] Helper variable template

template<class... B> inline constexpr bool disjunction_v = disjunction<B...>::value; |  | (since C++17)

### [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<bool(B1::value), B1, disjunction<Bn...>>  { };

### [edit] 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, 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.

### [edit] Example

Run this code

    #include <iostream>
    #include <type_traits>

    using result0 =
        std::disjunction<std::bool_constant<false>, std::bool_constant<false>,
                         std::bool_constant<false>>;
    using result1 =
        std::disjunction<std::bool_constant<true>, std::bool_constant<false>,
                         std::bool_constant<false>>;

    int main()
    {
        std::cout << std::boolalpha;
        std::cout << result0::value << '\n';
        std::cout << result1::value << '\n';
    }

Output:

    false
    true

### [edit] See also

negation (C++17) | logical NOT metafunction (class template) [edit]
conjunction (C++17) | variadic logical AND metafunction (class template) [edit]

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