Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 2/2016, seestd::disjunction(since C++17)

Forms thelogical disjunction of the type traitsB..., effectively performing a logical or on the sequence of traits.

The specializationstd::experimental::disjunction<B1, ..., BN> has a public and unambiguous base that is

• ifsizeof...(B)==0,std::false_type; otherwise
• the first typeBi inB1, ..., BN for whichbool(Bi::value)==true, orBN if there is no such type.

The member names of the base class, other thandisjunction andoperator=, are not hidden and are unambiguously available indisjunction.

Disjunction is short-circuiting: if there is a template type argumentBi withbool(Bi::value)!=false, then instantiatingdisjunction<B1, ..., BN>::value does not require the instantiation ofBj::value forj > i.

[edit]Template parameters

[edit] Helper variable template

[edit]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...>>{};

[edit]Notes

A specialization ofdisjunction does not necessarily inherit from of eitherstd::true_type orstd::false_type: it simply inherits from the firstB whose::value, explicitly converted tobool, is true, or from the very last B when all of them convert to false. For example,disjunction<std::integral_constant<int,2>,std::integral_constant<int,4>>::value is2.

[edit]Example

[edit]See also