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std::disjunction

From cppreference.com
< cpp‎ | types
 
 
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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 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

The behavior of a program that adds specializations for disjunction or disjunction_v 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>
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.

Feature testing macro: __cpp_lib_logical_traits

[edit] Example

#include <type_traits>
#include <cstdint>
#include <string>
 
// 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() { }

[edit] See also

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