std::conjunction

 

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Defined in header <type_traits>
template<class... B> struct conjunction;		(since C++17)

Forms the logical conjunction of the type traits B..., effectively performing a logical AND on the sequence of traits.

The specialization std::conjunction<B1, ..., BN> has a public and unambiguous base that is

• if sizeof...(B) == 0, std::true_type; otherwise
• the first type Bi in B1, ..., BN for which bool(Bi::value) == false, or BN if there is no such type.

The member names of the base class, other than conjunction and operator=, are not hidden and are unambiguously available in conjunction.

Conjunction is short-circuiting: if there is a template type argument Bi with bool(Bi::value) == false, then instantiating conjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for j > i.

If the program adds specializations for std::conjunction or std::conjunction_v, the behavior is undefined.

Template parameters

B...

-

every template argument Bi for which Bi::value is instantiated must be usable as a base class and define member value that is convertible to bool

Helper variable template

template<class... B> inline 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<bool(B1::value), conjunction<Bn...>, B1> {};

Notes

A specialization of conjunction does not necessarily inherit from either std::true_type or std::false_type: it simply inherits from the first B whose ::value, explicitly converted to bool, is false, or from 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.

The short-circuit instantiation differentiates conjunction from fold expressions: a fold expression like (... && Bs::value) instantiates every B in Bs, while std::conjunction_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

Run this code

#include <iostream>
#include <type_traits>
 
// func is enabled if all Ts... have the same type as T
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...) {
    std::cout << "all types in pack are T\n";
}
 
// otherwise
template<typename T, typename... Ts>
std::enable_if_t<!std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...) {
    std::cout << "not all types in pack are T\n";
}
 
int main() {
    func(1, 2, 3);
    func(1, 2, "hello!");
}

Output:

all types in pack are T
not all types in pack are T

See also

negation (C++17)	logical NOT metafunction (class template)
disjunction (C++17)	variadic logical OR metafunction (class template)