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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

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>
inline constexpr bool 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.

[edit] Example

#include <iostream>
#include <type_traits>
 
using result0 =
    std::disjunction<std::bool_constant<false>, std::bool_constant<false>,
                     std::bool_constant<false>>;
using result1 =
    std::disjunction<std::bool_constant<true>, std::bool_constant<false>,
                     std::bool_constant<false>>;
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << result0::value << '\n';
    std::cout << result1::value << '\n';
}

Output:

false
true

[edit] See also

(C++17)
logical NOT metafunction
(class template) [edit]
variadic logical AND metafunction
(class template) [edit]