std::disjunction (3) - Linux Manuals

std::disjunction: std::disjunction

NAME

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() { }