std::conjunction (3) - Linux Manuals

std::conjunction: std::conjunction

NAME

std::conjunction - std::conjunction

Synopsis

Defined in header <type_traits>
template<class... B> (1) (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.

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.

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