std:: 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 .

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

Template parameters

Helper variable template

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

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

See also