std::conjunction

Forms the logical conjunction of the type traits B...

The specialization std::conjunction<B1, ..., BN>

• if sizeof...(B) == 0, std::true_type
• the first type Bi in B1, ..., BN for which bool(Bi::value) == false , or BN

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

Helper variable template

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...>

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!");
}

All types in pack are the same.
Not all types in pack are the same.

See also