Difference between revisions of "cpp/types/disjunction"

Revision as of 03:50, 4 May 2017

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

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 <iostream>
#include <type_traits>
 
using result0 =
    std::disjunction<std::bool_constant<false>, std::bool_constant<false>,
                     std::bool_constant<false>>;
using result1 =
    std::disjunction<std::bool_constant<true>, std::bool_constant<false>,
                     std::bool_constant<false>>;
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << result0::value << '\n';
    std::cout << result1::value << '\n';
}

false
true

See also