Difference between revisions of "cpp/types/disjunction"

Revision as of 02:52, 28 March 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... B> struct disjunction
    : integral_constant<bool, (B::value || ...)>;

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, 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.

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