std::disjunction (3) Linux Manual Page

std::disjunction – std::disjunction

Synopsis

Defined in header<type_traits>

    template <class... B>
    (1)(since C++ 17)

        struct disjunction;

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

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