NAME

std::disjunction - std::disjunction

Synopsis

Defined in header <type_traits>
template<class... B> (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

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

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.

Feature-test macro: __cpp_lib_logical_traits

Example

// Run this code

#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>);