std::conjunction(3)

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.
   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 > (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 Value Std Feature
   __cpp_lib_logical_traits 201510L (C++17) Logical operator type 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";
 }
 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!");
 }

Output:
 all types in pack are T
 not all types in pack are T

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