std::experimental::conjunction
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 | Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 2/2016, seestd::conjunction(since C++17) |
Defined in header <experimental/type_traits> | ||
template<class...B> struct conjunction; | (library fundamentals TS v2) | |
Forms thelogical conjunction of the type traitsB..., effectively performing a logical AND on the sequence of traits.
The specializationstd::experimental::conjunction<B1, ..., BN> has a public and unambiguous base that is
- ifsizeof...(B)==0,std::true_type; otherwise
- the first type
BiinB1, ..., BNfor whichbool(Bi::value)==false, orBNif there is no such type.
The member names of the base class, other thanconjunction andoperator=, are not hidden and are unambiguously available inconjunction.
Conjunction is short-circuiting: if there is a template type argumentBi withbool(Bi::value)==false, then instantiatingconjunction<B1, ..., BN>::value does not require the instantiation ofBj::value forj> i.
Contents |
[edit]Template parameters
| B... | - | every template argumentBi for whichBi::value is instantiated must be usable as a base class and define membervalue that is convertible tobool |
[edit] Helper variable template
template<class...B> constexprbool conjunction_v= conjunction<B...>::value; | (library fundamentals TS v2) | |
[edit]Possible implementation
template<class...>struct conjunction:std::true_type{};template<class B1>struct conjunction<B1>: B1{};template<class B1,class...Bn>struct conjunction<B1, Bn...>:std::conditional_t<bool(B1::value), conjunction<Bn...>, B1>{}; |
[edit]Notes
A specialization ofconjunction does not necessarily inherit from eitherstd::true_type orstd::false_type: it simply inherits from the first B whose ::value, converted to bool, is false, or from the very last B when all of them convert to true. For example,conjunction<std::integral_constant<int,2>,std::integral_constant<int,4>>::value is4.
[edit]Example
#include <experimental/type_traits>#include <iostream> // func is enabled if all Ts... have the same typetemplate<typename T,typename...Ts>constexprstd::enable_if_t<std::experimental::conjunction_v<std::is_same<T, Ts>...>>func(T, Ts...){std::cout<<"All types are the same.\n";} template<typename T,typename...Ts>constexprstd::enable_if_t<!std::experimental::conjunction_v<std::is_same<T, Ts>...>>func(T, Ts...){std::cout<<"Types differ.\n";} int main(){ func(1,2'7, 3'1); func(1,2.7,'3');}
Output:
All types are the same.Types differ.
[edit]See also
(C++17) | variadic logical AND metafunction (class template)[edit] |
