# std::disjunction

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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`

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

### 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, 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.

### Example

Run this code

    #include <type_traits>
    #include <string>

    // checking if Foo is constructible from double will cause a hard error
    struct Foo {
        template<class T>
        struct sfinae_unfriendly_check { static_assert(!std::is_same_v<T, double>); };

        template<class T>
        Foo(T, sfinae_unfriendly_check<T> = {} );
    };

    template<class... Ts>
    struct first_constructible {
        template<class T, class...Args>
        struct is_constructible_x : std::is_constructible<T, Args...> {
            using type = T;
        };
        struct fallback {
            static constexpr bool value = true;
            using type = void; // type to return if nothing is found
        };

        template<class... Args>
        using with = typename std::disjunction<is_constructible_x<Ts, Args...>...,
                                               fallback>::type;
    };

    // OK, is_constructible<Foo, double> not instantiated
    static_assert(std::is_same_v<first_constructible<std::string, int, Foo>::with<double>,
                                 int>);

    static_assert(std::is_same_v<first_constructible<std::string, int>::with<>, std::string>);
    static_assert(std::is_same_v<first_constructible<std::string, int>::with<const char*>,
                                 std::string>);
    static_assert(std::is_same_v<first_constructible<std::string, int>::with<void*>, void>);

    int main() { }

### See also

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

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