std::disjunction - cppreference.com (original) (raw)

| | | | | ------------------------------------------- | | ------------- | | 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.

If the program adds specializations for std::disjunction or std::disjunction_v, the behavior is undefined.

Contents

• 1 Template parameters
• 2 Helper variable template
• 3 Possible implementation
• 4 Notes
• 5 Example
• 6 See also

[edit] Template parameters

[edit] Helper variable template

| template< class... B > constexpr bool disjunction_v = disjunction<B...>::value; | | (since C++17) | | ----------------------------------------------------------------------------------- | | ------------- |

[edit] Possible implementation

template<class...> struct disjunction : std::false_type {};   template struct disjunction : 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 #include #include   // 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::value is always true template struct default_type : std::true_type { using type = T; };   // Now we can use disjunction like a switch statement: template 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 // 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 struct sfinae_unfriendly_check { static_assert(<T, double>); };   template Foo(T, sfinae_unfriendly_check = {}); };   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, 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