Defined in header <type_traits> | ||
---|---|---|
template<class... B> struct conjunction; | (1) | (since C++17) |
Forms the logical conjunction of the type traits B...
.
The BaseCharacteristic of a specialization std::conjunction<B1, ..., BN>
is the first Bi
for which Bi::value == false
, or if every Bi::value != false
, the BaseCharacteristic is BN
.
If sizeof...(B) == 0
, the BaseCharacteristic is std::true_type
.
Conjunction is short-circuiting: if there is a template type argument Bi
with Bi::value == false
, then instantiating conjunction<B1, ..., BN>::value
does not require the instantiation of Bj::value
for j > i
.
Template parameters
B... | - | every type must be usable as a base class and define member B::value that is convertible to bool |
Helper variable template
template<class... B> constexpr bool conjunction_v = conjunction<B...>::value; | (since C++17) |
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<B1::value != false, conjunction<Bn...>, B1> {}; |
Notes
A specialization of conjunction does not necessarily have a BaseCharacteristic of either std::true_type
or std::false_type
: it simply inherits the base characteristic of the first B whose ::value, converted to bool, is false, or the base characteristic of the very last B when all of them convert to true. For example, std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value
is 4
.
Example
// func is enabled if all Ts... have the same type template<typename T, typename... Ts> std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...> > func(T, Ts...) { // TODO somethng to show }
See also
(C++17) | logical NOT metafunction (class template) |
(C++17) | variadic logical OR metafunction (class template) |
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