std::conjunction
Header: <type_traits>
Forms the logical conjunction of the type traits B…, effectively performing a logical AND on the sequence of traits.
# Declarations
template< class... B >
struct conjunction;
(since C++17)
# 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
#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 the same.\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 the same.\n";
}
template<typename T, typename... Ts>
constexpr bool all_types_are_same = std::conjunction_v<std::is_same<T, Ts>...>;
static_assert(all_types_are_same<int, int, int>);
static_assert(not all_types_are_same<int, int&, int>);
int main()
{
func(1, 2, 3);
func(1, 2, "hello!");
}