C++ : Folds over variadic templates

C++ : Folds over variadic templates

Code like the following motivates the need to compute conjunctions (and disjunctions) of predicate packs.

template <class T, class... Ts>
struct recursive_union {

  // ...

  //'U' is not 'T' but 'T' is a recursive wrapper and 'U' is the type
  //contained in 'T'
  template <class U, class... Args,
  std::enable_if_t<
     and_<
      is_recursive_wrapper<T>
    , std::is_same<U, unwrap_recursive_wrapper_t<T>>>::value, int> = 0
  >
  explicit recursive_union (constructor<U>, Args&&... args)
    noexcept (std::is_nothrow_constructible<U, Args...>::value)
  : v (std::forward<Args>(args)...)
  {}

  // ...

};

I was much helped by a suitable implementation of and_<> credited to Jonathan Wakely. A more general approach to is to use fold.

#include <type_traits>

namespace pgs {

template<class F, class Acc, class... Ts>
struct fold_left : Acc {
};

template <class F, class Acc, class T, class... Ts>
struct fold_left<F, Acc, T, Ts...> : 
    fold_left <F, typename F::template apply<Acc, T>::type, Ts...> {
};

//or

struct or_helper {
  template <class Acc, class T>
  struct apply : std::integral_constant<bool, Acc::value || T::value> {
  };
};

template <class... Ts>
struct or_ : fold_left <or_helper, std::false_type, Ts...> {
};

//and

struct and_helper {
  template <class Acc, class T>
  struct apply : std::integral_constant<bool, Acc::value && T::value> {
  };
};

template <class... Ts>
struct and_ : fold_left <and_helper, std::true_type, Ts...> {
};

}//namespace pgs