std::disjunction
Defined in header <type_traits> | ||
template<class... B> struct disjunction; | (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
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 |
[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> constexprbool 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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_logical_traits | 201510L | (C++17) | Logical operator type traits |
[edit]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 truetemplate<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 errorstruct 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 {staticconstexprbool 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<constchar*>, std::string>); static_assert(std::is_same_v<first_constructible<std::string, int>::with<void*>, void>); int main(){}
[edit]See also
(C++17) | logical NOT metafunction (class template) |
(C++17) | variadic logical AND metafunction (class template) |
