std::disjunction(3) C++ Standard Libary std::disjunction(3) NAME std::disjunction - std::disjunction Synopsis Defined in header <type_traits> template< class... B > (since C++17) struct disjunction; Forms the logical disjunction of the type traits B..., effectively per- forming a logical OR on the sequence of traits. The specialization std::disjunction<B1, ..., BN> has a public and unam- biguous 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 opera- tor=, 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. If the program adds specializations for std::disjunction or std::dis- junction_v, the behavior is undefined. 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 ei- ther 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_con- stant<int, 2>, std::integral_constant<int, 4>>::value is 2. The short-circuit instantiation differentiates disjunction from fold ex- pressions: a fold expression like (... || Bs::value) instantiates every B in Bs, while std::disjunction_v<Bs...> stops instantiation once the value can be de- termined. This is particularly useful if the later type is expensive to instantiate or can cause a hard error when instantiated with the wrong type. Feature-test macro Value Std Feature __cpp_lib_logical_traits 201510L (C++17) Logical operator type traits Example // Run this code #include <cstdint> #include <string> #include <type_traits> // values_equal<a, b, T>::value is true if and only if a == b. template<auto V1, decltype(V1) V2, typename T> struct values_equal : std::bool_constant<V1 == V2> { using type = T; }; // default_type<T>::value is always true template<typename T> struct default_type : std::true_type { using type = T; }; // Now we can use disjunction like a switch statement: template<int I> using int_of_size = typename std::disjunction< // values_equal<I, 1, std::int8_t>, // values_equal<I, 2, std::int16_t>, // values_equal<I, 4, std::int32_t>, // values_equal<I, 8, std::int64_t>, // default_type<void> // must be last! >::type; static_assert(sizeof(int_of_size<1>) == 1); static_assert(sizeof(int_of_size<2>) == 2); static_assert(sizeof(int_of_size<4>) == 4); static_assert(sizeof(int_of_size<8>) == 8); static_assert(std::is_same_v<int_of_size<13>, void>); // 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, dou- ble>); }; 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 (C++17) (class template) conjunction variadic logical AND metafunction (C++17) (class template) http://cppreference.com 2024.06.10 std::disjunction(3)
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