std::disjunction
| Defined in header <type_traits>
|
||
| template< class... B > struct disjunction; |
(since C++17) | |
Forms the [[enwiki|Logical disjunction|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
BiinB1, ..., BNfor which bool(Bi::value) == true, orBNif 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.
If the program adds specializations for std::disjunction or std::disjunction_v, the behavior is undefined.
Contents |
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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_logical_traits |
201510L | (C++17) | Logical operator type traits |
Example
#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, 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
| (C++17) |
logical NOT metafunction (class template) |
| (C++17) |
variadic logical AND metafunction (class template) |
