NAME

std::conjunction - std::conjunction

Synopsis

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

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.

The behavior of a program that adds specializations for conjunction or conjunction_v
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> (since C++17)
inline constexpr bool conjunction_v = conjunction<B...>::value;

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.

Feature-test macro: __cpp_lib_logical_traits

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